import java.util.*;
import java.io.*;

public class E74 {
    public static void main(String[] args) {
        MyScanner sc = new MyScanner();
        PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
        int n = sc.nextInt(); int m = sc.nextInt();
        String s = sc.next();
        long time = System.currentTimeMillis();
        int [][] a = new int[m][m];
        int [][] pre = new int[m][(1 << m)];
        for (int i = 0; i < n - 1; i++) {
            if (s.charAt(i) == s.charAt(i + 1)) continue;
            a[s.charAt(i) - 'a'][s.charAt(i + 1) - 'a']++;
            a[s.charAt(i + 1) - 'a'][s.charAt(i) - 'a']++;
        }
        //System.out.println("Process Input: " + (System.currentTimeMillis() - time));
        for (int i = 0; i < m; i++) {
            int b = 0; int stor = 2;
            for (int j = 1; j < (1 << m); j++) {
                if (j == stor) {
                    b++;
                    stor = (1 << (b + 1));
                }
                pre[i][j] = pre[i][j ^ (1 << b)] + a[i][b];
            }
        }
        //System.out.println("Precomp: " + (System.currentTimeMillis() - time));
        long [] dp = new long[1 << m];
        Arrays.fill(dp, Integer.MAX_VALUE);
        dp[0] = 0;
        for (int mask = 1; mask < (1 << m); mask++) {
            // loop on the last added character in the mask
            for (int i = 0; i < m; i++) {
                if (((mask >> i) & 1) == 0) continue;
                long prev = dp[mask ^ (1 << i)];
                long contribution = (pre[i][mask] - pre[i][((1 << m) - 1) ^ mask]) * Integer.bitCount(mask);
                dp[mask] = Math.min(dp[mask], prev + contribution);
            }
        }
        //System.out.println("Done: " + (System.currentTimeMillis() - time));
        out.println(dp[(1 << m) - 1]);
        out.close();
    }

    //-----------MyScanner class for faster input----------
    public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;

        public MyScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

    }

}


import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class E implements Runnable {
	public static void main (String[] args) {new Thread(null, new E(), "_cf", 1 << 28).start();}

	int n, m;
	char[] str;
	int[][] occs, cost;
	int[] dp;

	public void run() {
		FastScanner fs = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		System.err.println("");

		//where's my 420???? :(
		long tot = 0;
		for(int i = 0; i < 19999; i++) tot += i;
		System.err.println(tot);

		n = fs.nextInt(); m = fs.nextInt();
		byte[] str = fs.next().getBytes();
		int[] occs = new int[1<<m];
		for(int i = 0; i < n-1; i++) {
			int l1 = str[i] - 'a';
			int l2 = str[i+1] - 'a';
			occs[(1<<l1) | (1<<l2)]++;
			occs[(1<<l2) | (1<<l1)]++;
		}
		//cost[mask][v] = numPairs with v for some all bits on in mask
		int all = (1<<m)-1;
		cost = new int[m][1<<m];
		for(int i = 0; i < m; i++) {
			for(int mask = 1; mask < all; mask++) {
				if(((1<<i)&mask) > 0) continue;
				int lb = mask & (-mask);
				int trail = Integer.numberOfTrailingZeros(lb);
				int nmask = mask ^ lb;
				cost[i][mask] = cost[i][nmask]+occs[1<<i | 1<<trail];
			}
		}

		dp = new int[1<<m];
		for(int mask = dp.length-2; mask >= 0; mask--) {
			int addOn = 0;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				addOn += cost[nxt][mask];
			}
			int res = oo;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				int ret = addOn+dp[mask | (1<<nxt)];
				res = min(res, ret);
			}
			dp[mask] = res;
		}

		System.out.println(dp[0]>>1);

		out.close();
	}

	int oo = (int)1e9;
	int min(int a, int b) {
		if(a < b) return a;
		return b;
	}

	class FastScanner {
		public int BS = 1<<16;
		public char NC = (char)0;
		byte[] buf = new byte[BS];
		int bId = 0, size = 0;
		char c = NC;
		double num = 1;
		BufferedInputStream in;

		public FastScanner() {
			in = new BufferedInputStream(System.in, BS);
		}

		public FastScanner(String s) {
			try {
				in = new BufferedInputStream(new FileInputStream(new File(s)), BS);
			}
			catch (Exception e) {
				in = new BufferedInputStream(System.in, BS);
			}
		}

		public char nextChar(){
			while(bId==size) {
				try {
					size = in.read(buf);
				}catch(Exception e) {
					return NC;
				}
				if(size==-1)return NC;
				bId=0;
			}
			return (char)buf[bId++];
		}

		public int nextInt() {
			return (int)nextLong();
		}

		public long nextLong() {
			num=1;
			boolean neg = false;
			if(c==NC)c=nextChar();
			for(;(c<'0' || c>'9'); c = nextChar()) {
				if(c=='-')neg=true;
			}
			long res = 0;
			for(; c>='0' && c <='9'; c=nextChar()) {
				res = (res<<3)+(res<<1)+c-'0';
				num*=10;
			}
			return neg?-res:res;
		}

		public double nextDouble() {
			double cur = nextLong();
			return c!='.' ? cur:cur+nextLong()/num;
		}

		public String next() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c>32) {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public String nextLine() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c!='\n') {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public boolean hasNext() {
			if(c>32)return true;
			while(true) {
				c=nextChar();
				if(c==NC)return false;
				else if(c>32)return true;
			}
		}

		public int[] nextIntArray(int n) {
			int[] res = new int[n];
			for(int i = 0; i < n; i++) res[i] = nextInt();
			return res;
		}

	}

}


// No sorcery shall prevail. //
import java.util.*;
import java.io.*;
public class InVoker {
	//Variables
	static long mod = 1000000007;
	static long mod2 = 998244353;
	static FastReader inp= new FastReader();
	static PrintWriter out= new PrintWriter(System.out);
	public static void main(String args[]) {
	   	InVoker g=new InVoker();
	   	g.main();
	   	out.close();
	}

	int dp[];
	int freq[][];
	int m;

	//Main
	void main() {

		int n=inp.nextInt();
		m=inp.nextInt();
		String s=inp.next();
		dp=new int[1<<m];
		Arrays.fill(dp, -1);
		freq=new int[m][m];
		for(int i=0;i<n-1;i++) {
			int x=s.charAt(i)-'a';
			int y=s.charAt(i+1)-'a';
			if(x==y) continue;
			freq[x][y]++;
			freq[y][x]++;
		}
		out.println(go(0,0));

	}

	int go(int pos, int mask) {
		if(pos==m) return 0;
		if(dp[mask]!=-1) return dp[mask];
		int gg=Integer.MAX_VALUE;
		for(int i=0;i<m;i++) {
			if((mask>>i&1)==1) continue;
			int cost=0;
			for(int j=0;j<m;j++) {
				if((mask>>j&1)==1) cost+=freq[i][j]*pos;
				else cost-=freq[i][j]*pos;
			}
			cost+=go(pos+1,mask|(1<<i));
			gg=Math.min(gg, cost);
		}
		return dp[mask]=gg;
	}

	/*********************************************************************************************************************************************************************************************************
	 * ti;.                               .:,:i:         :,;;itt;.        fDDEDDEEEEEEKEEEEEKEEEEEEEEEEEEEEEEE###WKKKKKKKKKKKKKKKKKKKKWWWWWWWWWWWWWWWWWWW#WWWWWKKKKKEE      :,:.    f::::. .,ijLGDDDDDDDEEEEEEE*
	 *ti;.                               .:,:i:          .:,;itt;:      GLDEEGEEEEEEEEEEEEEEEEEEDEEEEEEEEEEE#W#WEKKKKKKKKKKKKKKKKKKKKKKKWWWWWWWWWWWWWWWWWWWWWWKKKKKKG.     .::.      f:,...,ijLGDDDDDDDDEEEEEE *
	 *ti;.                               .:,:i:            :,;;iti,    :fDDEEEEEEEEEEEEEEEKEEEEDEEEEEEEEEEEW##WEEEKKKKKKKKKKKKKKKKKKKKKWWWWWWWWWWWWWWWWWWWWWWWKKKKKKEG     .::.       .f,::,ijLGDDDDDDDDEEEEEE *
	 *ti;.                               .:,:i:             .,,;iti;.  LDDEEEEEEEEEEKEEEEWEEEDDEEEEEEEEEEE#WWWEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKWWWWWWWWWWWWWWWWWWWKKKKKEDj    .::.        .:L;;ijfGDDDDDDDDDEEEEE *
	 *ti;.                               .:,:i:              .:,;;iii:LLDEEEEEEEEEEEKEEEEEEEEDEEEEEEEEEEEW#WWEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKWWKWWWWWWWWWWWWWWKKKKKKKEL    .::.         .:;LijLGGDDDDDDDDEEEEE *
	 *ti;.                               .:,:;:                :,;;ittfDEEEEEEEEEEEEEEEEKEEEGEEEEEEEEEEEKWWWEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKWWWWWWWWWWWWWWWKKKKKKKELj   .::.          :,;jffGGDDDDDDDDDEEEE *
	 *ti;.                               .:,:i:                 .,;;tGGDEEEEEEEEEEEKEEEKEEEDEEEEEEEEEEEEWWWEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKWWWWWWKWWWWWWKKKKKKKEEL   .::.          .:;itDGGDDDDDDDDDEEEE *
	 *ti;.                               .:::;:                   :;ifDEEEEEEEEEEEEKEEEKEEEEEEEEEEEEEEEWWWEEEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKW#WWWKKKKKKKKEEf   .::.           :,itfGEDDDDDDDDDDDEE *
	 *ti;.                               .:::;:                    :GGEEEEEEEEEEEKEKEEKEEEEEEEEEEEEEEEEWWEEEEEEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKW#WKKKKKKKKKEEDG  .::.           .,;jfLGKDLDDDDDDEEDD *
	 *ti;.                               .:::;:                     fDEEEEEEKKKKKKKKKEKEEEEEEEEEEEEEEE#WEEEEEEEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKW#KKKKKKKKKKEEf .:::.           .,;tfLGDEDDDDDDDDEEE *
	 *ti;.                                :::;:                    fDEEEEEEKKKKKKKKKKWKEEEEEEEEEEEEEEEWKEEEEEEEEEEEEEEEEEEEEKEKKKKKKKKKKKKKKKKKKKKKKKKKKKKW##KKKKKKKKKEEft :::.           .,;tfLGDDDKDDDDDDDDD *
	 *ti;.                                .::;:                    fDEEEEEEKKKKKKKWKKKKKEEEEEEEEEEEEE#WEEWEEEEEDEEDEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKW#WKKKKKKKKEEGG :,:.           .,;tfLGGDDDKDDDDDDDD *
	 *ti;.                                .:.;:                   tGDEEEEKKKKKKKKKKKKKKKKKEEEEEEEEEEEWEEKWEEEEEEEDEEEEEEEEEEEEEEKEKKKKKKKKKKKKKKKKKKKKKKKKKKWWWKKKKKKKEEDf :::.           .,;tfLGGDDDDEDDDDDDD *
	 *ti;.                                .::;:                   fDEEEEEKKKKKKKKKKKWKKKKKKKKEEEEEEEWWEEWEEEEEEEEEEEEEEEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKW##KKKKKKKEEEft.::.           .,;tfLGGDDDDDDEDDDDD *
	 *ti;.                                .:.;:                  tGDEEEKKKKKKKKKKKKKKKKKKKKKKEKEEEEE#EEWWEEEEEEEEEEEEEEEEEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKW#WKKKKKKEEEGD:::.           .,;tfLGGDDDDDDDEDDDD *
	 *ti;.                                .:.,.                  LDEEEEKKKKKKKKKKWKWKKKKKKKKKKKKEEEKWEKW#EEEEEEEEEEEEEEEEKEEEEEEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKW##KKKKKKEEEEf,,:.           .,;tfLGGDDDDDDDDEDDD *
	 *ti;.                               ..:.,.                 LGDEEEEKKKKKKKKKKWKKKKKKKKKKKKKKKKKWEEW#WEEEEEEEEEEEEEEEKEEEEEEEEEEEEEEEEEEEKEKKKKKKKKKKKKKKKK##KKKKKEEEEEfi;,.           .,;tfLGGDDDDDDDDDKDD *
	 *tt;.                                .:.,:                 jDEEEEKKKKKKKKKKWWKKKKKKKKKKKKKKKKKWKE#WWEEEEEEEEEEEEEEWEEEEEEEEEEEEEEEEEEEEEEEKKKKKKKKKKKKKKKKWWKKKKEEEEDfG;,:           .,;tfLGGDDDDDDDDDDKD *
	 *tii,.                               .:.,.                tGDEEEEKKKKKKKKKKWWWKKKKKKKKKKKKKKKWKKWWWKEEEEEEEEEEEEEKEEEEEEEEEEEEEEEEEEEEEEEEEEEEEKKKKKKKKKKKW#KKKKEEEEDGGi;,.          .,;tfLGGDDDDDDDDDDDE *
	 *ti;;,:.                             .:.,:                fDEEEEKKKKKKKKKKKWKKKKKKKKKKKKKKKKKWEK#WWKEEEEEEEEEEEEDEEEEEEEEEEEEEEGEEEEEEEEEEEEEEEEEEEKKKKKKKWWKKEEEEEEDDf;;;,.         .,;tfLGGDDDDDDDDDDDD *
	 *tii;,,:..                           ...,.               ;LEEEEEKKKKKKWKKKKWKKKKKKKKKKKKKKKKKEKKW#WEEEEEEEEEEEEEjEEEEEEEEEKEEEEGEEEEEEEEEKEEEEEEEEEEEEEEEEE#WKEEEEEEDDf;;;;,:        .,itfLGGDDDDDDDDDDDD *
	 *ti;,,,,,:.                          ...,.               LDEEEEKKKKKKKKKKKWWWKKKKKKKKKKKKKKKWKK#W#WEEEEEEEEEEEDDLEEEEEEEEEWEEEEDEEEEEEEEEKEEEEEEEEEEEEEEEEEWWEEEEEEEDDfj,,,,,:.      .,itfGGGDDDDDDDDDDDD *
	 *tii,,,,::::.                        ...,:              .fDEEEEKKKKKKWKKKKWWWKKKKKKKKKKKKKKKEKKW#WWEEEEEEEEEEEKiKEEKEEEEEEWEEEEDEEEEEEEEEEEEEEEEEEEEEEEEEEEWWEEEEEEEDDLD:::,,,:.     .,ijfGGGDDDDDDDDDDDD *
	 *ti;:::::::::..                      .:.,:              LDEEEEKKKKKKKWKKKKWWKKKKKKKKKKKKKKKKtKKWWWWKEEEEEEEEEDiiDEEEEEEEEWWEEEEEEDEEEEEEEEEEEEEEEEEEEEEEEEEEWKEEEEEDDDGL:. .:,,,:    .,ijLGGGDDDDDDDDDDDD *
	 *tt;. .::::::::..                    ...,:             :fDEEEKKKKKKKKKKKKWW#KKKKKKKKKKKKKKKKfKKWWWWKEEEEEEEEDti,DEKEEEEEEWWEEEDEEEEEEEEEKEEEEEEEEEEEEEDEEEEE#WEEEEEGGDGf:.  .:,;,:.  .,ijLGGDDDDDDDDDDDDD *
	 *tt;.   .:::::::..                   ...,:             GDEEEKKKKKKKKWKKKKWWWKKKWKKKKKKKWWWKDEKLWWWWKKEEEEEEDEi,LDEEEEEEEEWWEEEEEEEEEEEEEEEEEEEEEEEEEDEDEEEEEW#EEEEDDDDGf,.    :,,,:...,ijLGGGDDDDDDDDDDDD *
	 *tt;.    .....::::..                 ...,:             fDEEEKKKKKKKKWKKKKWWWWKKKWKKKKKKKKKKfWKiWWW#KKEEEEEEEi;.EDfEEEDEEiWWEEEEEEEEEEEEDGKEEEEEEEEEEDEEEEEEEWWEEEEDDDGGLi.     .,;,:::,ijLGGGDDDDDDDDDDDD *
	 *tt;.      ....:::::.                ...,.            iDEEEEKKKKKKKKWKKWKWWWWWKKWWWKKKKKKKKtWKt#WWWKKEEEEEDji..DDKDDEDEGiWKEEEEEEEEEEDDEjEEEEEEEEEEEDEEEEEEEKWKEEDDDDGGff.      .:,;,,;ijLGGGDDDDDDDDDDDD *
	 *tt;.        ....::::..              .:.,:           .LDEEEKKKKKKKKKKKKWWWWKWWWWWWWWWWKKKKWtKKiDWWWKKKEEEEKi:..DEDDDDDDiiWKEEEEEEEEEEDDEijDEEEEEKEEEEEEEEEEEEWWEEGDDDGGLG.       .:,;;iijLGGGDDDDDDDDDDDD *
	 *tt;.          .....:::..            ...,.           .fEEEEKKKKKKKKWKKKKWWWWWWWWWWWWWWKWKKKiKDiLWWWWKEEEEEi,..fD:DDDDDti;WEEEEEEEEEEKDDi:iDDEEEEWEEEEEEEEEEEE#WEEGDDDDGGG.         :,iitjLGGGDDDDDDDDDDDD *
	 *tti.            .....:::..          ...,.           GDEEEKKKKKKKKKWKKKWWW#WWWWWWWWWWWKWKKjiEjitWWWKKWEEEDi...DDLDDDDji;;WEEEEEEEEEEEDEj.iDDEEEEWEEEEEEEEEEEEWWEEDDDDDDGf.          .,;tjfLGGDDDDDDDDDDDD *
	 *tti.              ....::::..        ...,.           fEEEKKKKKKKKKKKKKKKW#WWWWWWWWWWWWWWWWtiEiiiWWWKKEWKEi....D.EDDDEi;.fWEEEEEEEEEEDDfL.;EDDEEEWEEEEEEEEEEEEWWEEEDDDDDGf.           :;ijfLGGDDDDDDDDDDDD *
	 *tti.                ....::::..      ...,.          LDEEEKKKKKKKKKKKKKKWWWWWWWWWWWWWWWW####WKiiiWWWKKKEEK,...:E:DDDEii..GWEEEEEEEEDWDDiL.,KDDEEEWEEEEEEEEEEEEWWKEEDDDDDGf:           .,itfLGGDDDDDDDDDDDD *
	 *tti.                 .....:::..     ...,.          fDEEEKKKKKKKKKWKKKKWWWWWWWWWWWWW########WLiiWWWKKKEEjD...G,DDDDi;...EWEEEEEEEEDKDEii..LDDEEEWEEEEEEEEEEEEWWWEEDDDDDGfi           .,;tfLGGGDDDDDDDDDDD *
	 *tti.                   .....:::..   ...,.         iGEEEKKKKKKKKKKWKKKKWWWWWWWWWWWW###########KiWWWKKEEE,.D..D.DDDii:...KKEEEEEEEEEDDj:...tEDEEEWEEEEEEEEEEEEWWWEEEDDDDDLL           .,;tjLLGGDDDDDDDDDDD *
	 *tti.                     ....::::......:.         LEEEKKKKKKKKKKWWKKKWWW#KWWWWWWWW#####W####W##KWWKKEEL..:D.jjDDi;,....KKEEEEEEEDfDDi...:iKDEEEWKEEEEEEEEEEEWWWEEEEDDDDLG           .,;tjLLGGDDDDDDDDDDD *
	 *tti.                        ...::::::..,.        :GEEEKKKKKKKKKKKKWWWWW##WWWWWWWWW##WKWK#W#W####WWKEEK.....G.DDti,.....KKEEEEEEDWGDf.,...iKDEEEWWEEEEEEEEEEEW#WEEEEEDDDGL           .,;tjLLGGDDDDDDDDDDD *
	 *tti.                         ....::::::,.        GDEEKKKKKKKKKKKKKWWWW###WWWWWWWWWW#WWWK###W#####WKEKK.....jDDL;;......KKEEEEEEEEEDi.f...;KDEEEWWEEEEEEEEEEEWWWWEEEEEDDGf           .,;tjLLGGDDDDDDDDDDD *
	 *tti.                           ....:::,,.       .LEEEKKKKKWKKKKKWWWWWW###WWWWWWWWWW#WWKW#WW##W#WWWKEKD:....:DD:;......;KEEEEEEEKiDD..f...,KKEEEWWEEEEEEEEEEEWWWWEEEEEDDDf           .:;tjLLGGGDDDDDDDDDD *
	 *tti.                             ...::,,,:.     GDEEKKKKKKKKKKKKWWWWWWW#WWWWWWWWWWW#KjKWWWWWWWWWWWWEK.j,..;fD.;.......fKEEEEEDKG:Di..,....DKEEEWWEEEEEEKEKKKWWWWEEEEEEDDf           .:;tjLLGGDDDDDDDDDDD *
	 *jti.                              ...::,,,,:.  .fEEEKKKKKWKKKKKKWWWWWWW#WWWWWWWWWWK#KKKWWWWWWWWWWWWWK..f:.:G.,:.......EKEEEEEKK;:E:.......fKEEEWWKEKEKKKKKKKW#WWEEEEEEDDf:          .,;tfLLGGDDDDDDDDDDD *
	 *tti.                               ...:,,,;;,: iDEEKKKKKWKKKKKKKWWWWWWW#WWWWWWWWWWK#WDKWWKKWWWWWWWWWE..;G:G..,........KKEEEEEKi.Gi..:.....tKEEKWWWKKKKKKKKKKW##WKEEEEEEDfi          .,;tfLLGGGDDDDDDDDDD *
	 *tti.                               ....::,,;;;,LEEKKKKKKWKKKKKWWWWWWW###WWWWWWWWWWKWWDKWEEEWKKWWWWWKKj.:LG..;.........EKEEEEKG;.G...;.....;KKEKWWWKKKKKKKKKKW##WWKEEEEEDfL          .,;tfLGGGDDDDDDDDDDD *
	 *jti.                                ...::::,;ijDEEKKKKKWKKKKKKWKWWWWW##WWWWWWWWWWWKK#KKGDGDWEEWKKWKKGE,.i;.:.........:EKEEEKE;.:L...j.....,KWEKWWWKKKKKKKKKK####WKKEEEEDLG          .,;tfLGGGGDDDDDDDDDD *
	 *jti.                                ...:...,,;GEEKKKKKWWKKKKKWWWWWWWW###WWWWWWWWWKKKWWKiLGGEDEDEKGKKiEG..;...........jKEEEKK;:.G....,.....:KKEWWWWKKKKKKKWKK####WKKKKEEEGL          .,;tfLGGGGDDDDDDDDDD *
	 *jti.                                ...:.  .:,GEEKKKKKWKKKKKWWWWWWWW####WWWWWWWWWKKKWWKii;fDLGDK: EEi:E:.............EKEEKK;;..L...........KKKWWWWKKKKKKKWKK####WKKKWKEEDf          .,;tfGGGGDDDDDDDDDDD *
	 *jti.                                ...:.    ,EEKKKKKWWKKKKKWWWWWWWWW###WWWWWWWWKKKKfWWLt;i,. fi  EG..D:.............EKEKK;;..t....:.......KWKWWWWKKKKKKKWKK####WKKKWEEEDf:.        .,;tfGGGGDDDDDDDDDDD *
	 *jti.                                ...:.    GEEKKKKKWKKKKKWWWWWWWWW####WWWWWWWKKKKKt;KKEfff     .;t.................KKKKi;:..GtGGfG.......KWWWWWWKKKKKKKWKK###WWWKKKKEEEf,,:       .,;tfGGGGDDDDDDDDDDD *
	 *jti.                                ...:.    GEKKKKKWWKKKKKWWWWWWWWWW##WWWWWWWKKKKKKt;EiKKKK,   ...t................jEKKG;;..,.....,LGi....KWWWWWWKKKKKKWKKKW####WKKKKKEEL,,,:.     .,;tfGGGDDDDDDDDDDDD *
	 *jti.                                ...:.  .GEEKKKKKWKKKKKWWWWWWWWWW###WWWWWWWKKKKKKtiE::tGG........................EEEj;;...,.........:D..DKWWWWWWKKKKK#KKW###W#WKKKKKEEfj:,,,:.   .,;tfGGGDDDDDDDDDDDD *
	 *jti.                                ...:.   DEKKKKKWWKKKKKWWWWWWWWW####WWWWWWWKKKKKKiiE:::.::.......................EEi;;...j.....f......:iDKWWWWWWKKKKK#WW######WKKKKKEELG :,,,,:. .,;tfGGGDDDDDDDDDDDD *
	 *jti.                                ...:.  fEEKKKKWWKKKKWWWWWWWWWWW###WWWWWWWWKKKKKK;tE::..........................DD;.;,.::......;........EWWWWWWWKKKKW#WW#####WWKKKWKKELG  .:,,,:::,;tfGGGDDDDDDDDDDDD *
	 *jti.                                ...:. .DEKEKKKWWKKKKWWWWWWWWWWW###WWWWWWWWKKKKKE,iD::..........................D..,;.,;tLffi...........DWDWWWW#KKKWWWWW#####W#KKKWKEEGL    .:,;,,,;tfGGGDDDDDDDDDDDD *
	 *jti.                                ...:. ;EEKKKKWWKKKKKWWWWWW#WWWW####WWWWWWKKKKKEL:iD:..........................j ..;..;;:.....i,........DKtWWWWWKKWWWWWW#####WWWKKKEKEDf     .:,;;;itfGGGDDDDDDDDDDDD *
	 *jti.                                ...:. DEKKKKKWWKKKKWWWWWWW#WWWW####WWWWWWKKKKKEj:iG...............................:....................GKiWWWWWKKWW#WWW######WWKKKKKEEf       .,;iitfGGGDDDDDDDDDDDD *
	 *jti.                                ...:.:EKKKKKWWKKKKKWWWWWWW#WWW#####WWWWWKWKKKKEi:if:.................................iEKEKKKKKKDj......DKiWWWWWKWK##WW#######WWKKK:KEEL        .:;itfGGGDDDDDDDDDDDD *
	 *jji.                                ...:,DEEKKKWWWKWKKWWWWWWWWWWWW#####WWWWWWWKKKKEi:it..................................j.  KKKKKKKKKKKf..DKiWWWWWKWW##WW#######WWKKK,KEEf         .,;tfGGGDDDDDDDDDDDD *
	 *jji.                                ..L:iDEEKKKWWKKKKKWWWWWWWWWWWW#####WWWWWKWKKKKKi.i;..................................  . KKKWWWWWWWWK..DGiWWWWWKK##WWW#####W#WWKKKjEKEL,        .:;tfGGGDDDDDDDDDDDD *
	 *jji.                               .f:::EEEKKKWWWKKKKKWWWWWWWWWWWW#####WWWWWKWKKKKK;.i,.................................::  KKEKWWWWWWfWK..EiiWWWWWKWW#WW##########KKKD,KELj        .:;tfGGDDDDDDDDDDDDD *
	 *jji.                             .t::::,DEEKKKWWKKKKWWWWWWWWW#WWWW#####WWWWKKWKKKEK;.i:.................................GDDEEEKKKWWWWWtWWD.E;iWWWWWW###WW#########WWKKK.EEDG        .:;tfGGGDDDDDDDDDDDD *
	 *jji.                          . j..::::EKEKKKWWWKKKKWWWWWWWWW#WWW######WWWWKKWKKKEK;.t:.................................ELLEDDEEEWWWWEtWK,.KiiWWWWWW###W##########WWKKK:EEEG     .;tjfLLGDDDDDDDDDDDDDDD *
	 *jji.                          i.::::::,EEEKKWWWKKKKKWWWWWWWWW#WWW#####WWWWWKWKKKKEE,.t..................................DfiEGDDDEEKKKttKWG.KiiWWWWW##WWW##########WWKKK:fEEL  ,fGGGDDDDDDDDEEEDDDDDDDDDD *
	 *jji.                        .;:..:::::DEEEKKWWWKKKKKWWWWWWWWW#WWWW####WWWWWWWKKKKED,.t..................................ifjDDGGEGDKK.ttKKE.DiWWWWW###WW##########WWWKKK:.KELiLGGGGDDDDDDDDDDDDEEEDDDDDDD *
	 *jji.                       i.:.::::::,KEEKKWWWKKKKKKWWWWWWWWW#WWWW####WWWWWWWKKKKEL:.j..................................GGf,;ifLLED .iiKKi:fWWWWWW##W#W##########WWWKKK:.KKLGGDDDDDDDDDDDDDDDDEDDEEDDDDD *
	 *jji.                     .j:.::::::::EEEKKKWWWKKKKKKWWWWWWWW##WWW#####WWWWWWWKKKKKf:.f..................................:EEfftf .,. ;iE,..jWWWWWWW###W############WWKKK,:KKGDDDDDDDDDDDDDDDDDDDDDDDEDDDD *
	 *jji.                    .:.::::::::,,EEEKKWWWKKKKKKKWWWWWWWW##WWW#####WWWWWWWKKKKKt..G....................................EEELL;   .j....tKWWWWWWW################WWWKKtfGKGEDDDDDDDDDDDDDDDDDDDDDDDEEDD *
	 *jji.                   :...:::::::,,jEEKKWWWWKKKKKKWWWWWWWWW##KWW#####KWWWWWWKKKKEi..D....................................:jEEE.........;KKWWWWWWWW#WW##W##########WWKKDLGKEKDDDDDDDDDDDDDDDDDDDDDDDDDED *
	 *jji.                  i:.::::::::,,,EEEKKWWWWKKKKKWWWWWWWWWW##WWW#####WWWWWWWKKKKKi..D......................................:::::......,KKKWWWWWWWWW#####W########WWWKKKGGKKEGGGGGGGGDDDDDDDDDDDDDDDDDDE *
	 *jji.                 i..:::::::::,,tEEKKWWWWKKKKKWWWWWWWWWWW##WW######WWWWWWWKKKKKi..D......................................::::......:EKKKWWWWWWWWWWW##WW########W#WKKWGGKKGGGGGGGGGGGGGGGDDDDDDDDDDDDD *
	 *jji.                .:::::::::::,,,EEEKKWWWWKKKKKWWWWWWWWWWW##WW#####WWWWWWWWKKKKKi..D....................................:::::::::..tELii;KWWWWWWWWWW##WW######WWWWWWKWGGGKGGGGGGGGGGGGGGGGGGGGGGGGGGDG *
	 *jjt.                :.::::::::,,,,fEEKKWWWWKKKKKKWWWWWWWWWW###WW####WWWWWWW#WKKKKKi..D....................................:::::::.:.,;;;;;;,KKWWWWWWWWW#WW########WWWKKWGGGKGGGGGGGGGGGGGGGGGGGGGGGGGGGG *
	 *jji.               ;.::::::::,,,,;EEEKWWWWWKKKKKWWWWWWWWWWWW##WW###WKWWWWWK#WKKKKKi..G......................................:::::::,;;;;:...KKKWWWWWWWWWKWW#######WWWWKKGLGKDGGGGGGLLGGGGGGGGGGGGGGGGGGG *
	 *jjt.              f.:::::::::,,,,fEEKKWWWWWKKKKKWWWWWWWWWWW###WW##WKKWWWWWW#WKKKKK;.jt........i.............................:::::::;j;;....:E.KKKWWWWWWWKWW#####W#WWWWKKLLGWEEGGGGGLGGGGGGGGGGGGGGGGGGGG *
	 *jjt.             ...:::::::,,,,,;DEEKWWWWWKKKKKWWWWWWWWWWWW####WWWKKKWWWWWWWWKKKKK;.E;.........t.............................:::::ii;;.....D...KKWWWWWWWKWW#####WWEWWWKKGGGEKKGGGGGLGGGGGGGGGGGGGLGGGGGG *
	 *fji.             ;.:::::::,,,,,;LEEKKWWWWWKKKKKWWWWWWWWWWWW####KWKKKKWWWWWWWWKKKKKi.D;..........j.............................:::tt;,.....:.....KKWWWWWWKWWWW##WWWGWWWKKGGGGKEGGGGGGGGGGGGGGGGGGGLLGGGGL *
	 *fji.            t::::::::,,,,,,;EEEKWWWWWKKKKKKKWWWWWWWWWWW##WKWKKKKKWWWWWWWWKKKKKi:D;............j...........................::LL;,.............KKWWWWWKWWWWWWWWWGWWWKKGGGGKGGGGGGGGGGGGGGGGGGGGLLGGGGL *
	 *fjt:            .:::::::,,,,,,,DEEKWWWWWWKKKKKKKWWWWWWWWWWWWKKWKKKKKKWWWWK#WWKKKKWitE;........... ............................:G;;:...............KKKWWKKWWWWWWWWWGWWWKKGGGGWGGGGGGGGGGGGGGGGGGGGGGGGGGL *
	 *fjji;:.       .f:::::::,,,,,,,;EEEKWWWWWWKKKKKKWWWWWWWWWWWKKKKKKKKKKKWWKWWWWWKKKKWGKD;........................................L;;..................DKKWKKWWWWWWWWWGWWWKKDGGGKDGGGGGGGGGGGGGGGGGGGGGGGGGG *
	 *fjjtii;,:.     :::::::,,,,,,,;EEEKWWWWWWKKKKKKWWWWWWWWWWKKKKKKKKKKKKWWWWWW#WWKKKKWiEj;......................................:i,;....,...............;KKEKWWWWWWWWWGKWWKKDDGGDEGGGDGGGGGDGGGGGGGGGGGGGGGG *
	 *fjtiiiii;;:.  j::::::,,,,,,,;;EEEKWWWWW#KKKKKWWWWWWWWWKKKKKKWKKKKKKKWWWWWWWWWKKKKWtEL;:....................................;;;:...,;j................:KEEWWWWWWWWWDDWWKKDDDDDKDDDDDDDDDDDDDDDGGGGGGGGGGG *
	 *fjti;;iiii;;,:::::::,,,,,,,,;EEEKWWWWWWWKKKKWWWWWWWWKKKKKKKWKKKKKKKWWWWWWW#W#KKKKWEEii;...................................f;:....,;L...................EEKWWWWWWWWDDWWKKDDDDDKEDDDDDDDDDDDDDDDDDDDDDGGGG *
	 *fjt,,,;;;;ii;f::::::,,,,,,,;;EEKWWWWWWWKKKKKWWWKWWKKKKKKKKKKKKKKKKKWWWWWWW#W#KKKKWKEij;:...............................:G;,.....,;f....................:tKKWWWWWWWDDWWKKDDDDDKKDDDDDDDDDDDDDDDDDDDDDDDDD *
	 *jjt. ..:,;;;;,::::,,,,,,,,;;GEEWWWWWWWWKKKKWKKWKKKKKKKKKKKKKKKKKKKKWWWWWWW#W#KKKKWEDi;j;............................,Li;L;;;..,;;f........................KKKKWWWKDDWWKKDDDGDKKGGGGGGGGDGDDDDDDDDDDDDDDD *
	 *fjt.    .:,,,:::::,,,,,,,;;;EEKWWWWWWWKKKKKKWKKKKKKKKKKKKKKKKKKKKKWKKKWKW##W#KKKKWEti;;G;........................tEEEL;;;;;;;;;;L..........................DKKKKKEDDWWKEDGftiLE;;;;itjLGGGGGGDDDDDDDDDDD *
	 *fjt.       .j::::,,,,,,,;;;DEEWWWWWWWWKKKKWKKKKKKKKKKKKKKKKKKKKKKKWKKWWWK##W#KKKKKEii;;;L;...................iDEEEEEEKKi;j;;;;jD.....:......................,KKKKDGGEKKE:::::;E::::::::::,tLGGDDDDDDDDDD *
	 *fjt.       .;:::,,,,,,,;;;;EEKWWWWWWWWKWKKKKKKKKKKKKKKKWKKKKKKKKKKWKKWWWW#WW#KKKKKKii;;;;f;.............:tDEEEEEKKKKKKKKEti;;;L...............................EEKf;:iKKE::::::E::::::::::::::ifDDDDDDDDD *
	 *fjt:        :::,,,,,,,,;;;DEEWWWWWWWWWEKKKKKKKKKKKKKKKKKKKKKKKKKKWWKKWWWW####KKKKKEiii;;;;f,.........iDEEEEKKKKKKKKKKKKKKKf;iG......i..........................fK::::KKE::::::E::::::::::::::::,tGGDDDDD *
	 *fjt:       t:::,,,,,,;;;;iDEKWWWWWWKEKKKKKKKKKKKKKKKKKKKKKKKKKKKKWWKKWWWW####WKKKKLiii;;;;;L,....,Li;EDEEEEKKKKKKKKKKKKKKKKiG......;:...........................:i:::KKE:::::,E,::::::::::::::::::iGDDDD *
	 *jjt.       f::,,,,,,,;;;;GEEWWWWKEEKEKKKKKKKKKKKKKKKKWKKKKKKKKKKKWWKWWWWW###WWKKKKiii;;;;;;;G,;L;;iiEEEEEEEKKKKKKKKKKKKKWWKE......;t.........:....................j::KEE:,::,,D,,::::,,,,,,:::::::::tDDD *
	 *fjt:.      ,::,,,,,,,;;;;EEWWKEEEEEEKKKKKKKKKKKKKKKKWKKKKKKKKKKKWWKKWWWWW#W#KKKKKKiiiiii;;;;;i;;iiiEEKEEKKWKKKKKKKWKKKKKWWWGi;...;t......,;;;;,....................:,EEE,,,,,,D,,,,,,,,,,,,,,,,::,::::tG *
	 *fjt:.     ,::,,,,,,,;;;;DEKEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKWWWWWWWWW#W#KKKKKKiiiii;;i;;;;;iiiKKKEKKKKWWKWWWWWWKKKKWWWWW;;;:;L.....;;;;;;;;;....................,KEE,,,,,,E,,,,,,,,,,,,,,,,,,,,,,,,; *
	 *fjt:.     f:,,,,,,,;;;;jEDEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKWWWWWWWW#W##KKKKKKiiiiiiii;i;;iiiEKKKKKKKKWKWWWWWWWWKKKWWWWWKi;;i.....,jEEfGi;;;;;...................EED,,,,,,E,,,,,,,,,,,,,,,,,,,,,,,,, *
	 *fjt:.    .f::,,,,,,;;jEEDEEEEEEEEEEKKKKKKKKKKKKKKKWKKKKKKKKKKKKKWWWKWWWWW###KKKKKLiiiiiiiiiiiiiiEEKKKKKKKKWWWWWWWWWWWWKWWWWWWGi;i;,..;jDDDKEGi;;;;;;:................EED,,,,,,D,,,,,,,,,,,,,,,,,,,,,,,,, *
	 *fjt:.  .. ;::,,,,,;;EDDEEEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKWWKKW#WW####KWKKKiiiiiiiiiiiiijKKKKKKKKKKWWWWWWWWWWWWWWWWWWWWWt;i;;;;i;DDDDDDGi;;;;;;;;:.............EDf;,,,;,G;;;;;;;;;;;;;;;,,,,,,,,,, *
	 *fjt:......:,,,,,,;LDDDEEEEEEEEEEEKKKKKKKKKKKKKKKKWKKKKKKKKKKKKKWWWWKWWWW####KKKKKiiiiiiiiiiijKEKKWKKKKKKKWWWWWWWWWWWWWWWWWWWWWWiLiii;i;DEEEEDDE;i;;;;;;;;;:..........EDi,;;;;;L;;;;;;;;;;;;;;;;;;,,,,,,, *
	 *fjt:......:,,,,,;EDDDEEKEEEEEEEEEKKKKKKKKKKKKKKKWKKKKKKKKKKKKKKWWWWKKWWW##W#KWKKWEiiiiiijGKKKKKWWKKKKKKKKWWWWWWWWWWWWWWWWWWWWWWKi;iiiiDDEEEEEEDEi;;;;;;;;;;;;;,:.....ED;;;;;;;j;;;;;;;;;;;;;;;;;;;;;;;,, *
	 *fjt:.....t,,,,,;DDDDEEEKEEEEEEEEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKWWWKKKWWWW##WKWKKWKiiiKKKKKKKKKWWKKKKKKKKWWWWWWWWWWWWWWW#WWWWWWWWWiiiiifLEEEEEEEEDi;i;;;;;;;;;;;;.....DD;;;;;;;i;;;;;;;;;;;;;;;;;;;;;;;;; *
	 *fjt:.....G,,,,,GDDDEEEEEEEEEEEEKKKKKKKKKKKKKKKKWKKKKKKKKKKKKKKKWWWKKKWWW###WKWKKWKitKKKKKKKKKWKKKKKKKKKKWWWWWWWWWWWWWW###WWWWWWWWEiiiiiiiEEEEEEEEDGiiii;;;;;;;;;.....GD;;;;;;;i;;;;;;;;;;;;;;;;;;;;;;;;; *
	 *fjt:.....L,,,,;GDDDEEEEEEEEEEKEKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKWWWWWDGWWW###KKWWKWKKKKKKKKKKKKKKKKKKKKKKKWWWWWWWWWWWWW####WWWWWWWWWiiiiiiiiEEEEEEEEEEDi;i;;;;;;;;.....Lj;;;;;;i;iiiiii;;;;;;ii;;;;;;;;;;; *
	 ***********************************************************************************************************************************************************************************************************/

	void sort(int a[]) {
		ArrayList<Integer> list=new ArrayList<>();
		for(int x: a) list.add(x);
		Collections.sort(list);
		for(int i=0;i<a.length;i++) a[i]=list.get(i);
	}
	void sort(long a[]) {
		ArrayList<Long> list=new ArrayList<>();
		for(long x: a) list.add(x);
		Collections.sort(list);
		for(int i=0;i<a.length;i++) a[i]=list.get(i);
	}
	void ruffleSort(int a[]) {
		Random rand=new Random();
		int n=a.length;
		for(int i=0;i<n;i++) {
			int j=rand.nextInt(n);
			int temp=a[i];
			a[i]=a[j];
			a[j]=temp;
		}
		Arrays.sort(a);
	}
	void ruffleSort(long a[]) {
		Random rand=new Random();
		int n=a.length;
		for(int i=0;i<n;i++) {
			int j=rand.nextInt(n);
			long temp=a[i];
			a[i]=a[j];
			a[j]=temp;
		}
		Arrays.sort(a);
	}

    static class FastReader {
        BufferedReader br;
        StringTokenizer st;
        public FastReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }
        String next() {
            while (st==null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                }
                catch (IOException  e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
        int nextInt() {
            return Integer.parseInt(next());
        }
        long nextLong() {
            return Long.parseLong(next());
        }
        double nextDouble() {
            return Double.parseDouble(next());
        }
        String nextLine() {
            String s="";
            try {
                s=br.readLine();
            }
            catch (IOException e) {
                e.printStackTrace();
            }
            return s;
        }
    }

	long fact[];
	long invFact[];
	void init(int n) {
		fact=new long[n+1];
		invFact=new long[n+1];
		fact[0]=1;
		for(int i=1;i<=n;i++) {
			fact[i]=mul(i,fact[i-1]);
		}
		invFact[n]=power(fact[n],mod-2);
		for(int i=n-1;i>=0;i--) {
			invFact[i]=mul(invFact[i+1],i+1);
		}
	}

	long nCr(int n, int r) {
		if(n<r || r<0) return 0;
		return mul(fact[n],mul(invFact[r],invFact[n-r]));
	}

	long mul(long a, long b) {
		return a*b%mod;
	}
	long add(long a, long b) {
		return (a+b)%mod;
	}

	long power(long x, long y) {
		long gg=1;
		while(y>0) {
			if(y%2==1) gg=mul(gg,x);
			x=mul(x,x);
			y/=2;
		}
		return gg;
	}

    // Functions
	static long gcd(long a, long b) {
		return b==0?a:gcd(b,a%b);
	}
	static int gcd(int a, int b) {
		return b==0?a:gcd(b,a%b);
	}

	void print(int a[]) {
		int n=a.length;
		for(int i=0;i<n;i++) out.print(a[i]+" ");
	}

	void print(long a[]) {
		int n=a.length;
		for(int i=0;i<n;i++) out.print(a[i]+" ");
	}

    //Input Arrays
    static void input(long a[], int n) {
		for(int i=0;i<n;i++) {
			a[i]=inp.nextLong();
		}
	}
	static void input(int a[], int n) {
		for(int i=0;i<n;i++) {
			a[i]=inp.nextInt();
		}
	}
	static void input(String s[],int n) {
		for(int i=0;i<n;i++) {
			s[i]=inp.next();
		}
	}
	static void input(int a[][], int n, int m) {
		for(int i=0;i<n;i++) {
			for(int j=0;j<m;j++) {
				a[i][j]=inp.nextInt();
			}
		}
	}
	static void input(long a[][], int n, int m) {
		for(int i=0;i<n;i++) {
			for(int j=0;j<m;j++) {
				a[i][j]=inp.nextLong();
			}
		}
	}

}


import java.util.*;
import java.lang.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.*;
import java.math.*;
public class Main {
    public void dfs(ArrayList<Integer>[] graph,int[] visited,int source) {

    }
    public static void main(String[] args) throws Exception{
        Reader.init(System.in);
        PrintWriter out = new PrintWriter(System.out);
        Main mm =new Main();
        int n=Reader.nextInt();
        int m=Reader.nextInt();
        String s=Reader.next();
        int[][] count=new int[m][m];
        for(int i=1;i<n;i++) {
            count[s.charAt(i)-'a'][s.charAt(i-1)-'a']++;
            count[s.charAt(i-1)-'a'][s.charAt(i)-'a']++;
        }
        int[] dp=new int[1<<m];
        Arrays.fill(dp, Integer.MAX_VALUE/10);
        for(int i=0;i<m;i++) {
            dp[1<<i]=0;
        }
        for(int i=0;i<(1<<m);i++) {
            int extra=0;
            for(int j=0;j<m;j++) {
                if((i&(1<<j))>0) {
                for(int k=0;k<m;k++) {
                    if(j!=k &&  (i&(1<<k))==0) {
                        extra+=count[j][k];
                    }
                }
               }
            }
            for(int j=0;j<m;j++) {
                if((i&(1<<j))==0) {
                    dp[i|(1<<j)]=Math.min(dp[i|(1<<j)], dp[i]+extra);
                }
            }
        }
        out.println(dp[(1<<m)-1]);
        out.close();
}
}
class Reader {
    static BufferedReader reader;
    static StringTokenizer tokenizer;
    /** call this method to initialize reader for InputStream */
    static void init() throws IOException {
         reader = new BufferedReader(
                 new FileReader("input.txt"));
    tokenizer = new StringTokenizer("");
    }
    static void init(InputStream input) {
        reader = new BufferedReader(
                     new InputStreamReader(input) );
        tokenizer = new StringTokenizer("");
    }
    /** get next word */
    static String nextLine() throws IOException{
        return reader.readLine();
    }
    static String next() throws IOException {
        while ( ! tokenizer.hasMoreTokens() ) {
            //TODO add check for eof if necessary
            tokenizer = new StringTokenizer(
                   reader.readLine() );
        }
        return tokenizer.nextToken();
    }
    static int nextInt() throws IOException {
        return Integer.parseInt( next() );
    }
    static long nextLong() throws IOException {
        return Long.parseLong( next() );
    }
    static double nextDouble() throws IOException {
        return Double.parseDouble( next() );
    }
}


import java.io.*;
import java.util.*;

import static java.lang.Math.*;

public class E {
    static int g[][];
    static int n, m;
    static char[] s;
    static int dp[], inf = (int) 2e9;

    public static void main(String[] args) throws Exception {
        Scanner in = new Scanner(System.in);
        PrintWriter pw = new PrintWriter(System.out);
        n = in.nextInt();
        m = in.nextInt();
        s = in.next().toCharArray();
        g = new int[m][m];
        for (int i = 1; i < n; i++) {
            int x = s[i - 1] - 'a', y = s[i] - 'a';
            if (x != y) {
                g[x][y]++;
                g[y][x]++;
            }
        }
        dp = new int[1 << m];
        Arrays.fill(dp, -1);
        pw.println(solve(0, 0));

        pw.close();
    }

    static int solve(int pos, int mask) {
        if (pos >= m) return 0;
        if (dp[mask] != -1) return dp[mask];
        int min = inf;
        for (int i = 0; i < m; i++) {
            if (!check(mask, i)) {
                int res = 0;
                for (int j = 0; j < m; j++) {
                    if (check(mask, j)) res += g[i][j] * pos;
                    else res -= g[i][j] * pos;
                }
                res += solve(pos + 1, set(mask, i));
                min = min(min, res);
            }
        }
        return dp[mask] = min;
    }

    static boolean check(int N, int pos) {
        return (N & (1 << pos)) != 0;
    }

    static int set(int N, int pos) {
        return N = N | (1 << pos);
    }

    static int reset(int N, int pos) {
        return N = N & ~(1 << pos);
    }

    static void debug(Object... obj) {
        System.err.println(Arrays.deepToString(obj));
    }
}


import java.io.*;
import java.util.*;
import static java.lang.Math.*;

public class E_fast {
    static int g[][];
    static int n, m;
    static char[] s;
    static int dp[], inf = (int) 2e9;
    static int cost[][];

    public static void main(String[] args) throws Exception {
        InputReader in = new InputReader(System.in);
        PrintWriter pw = new PrintWriter(System.out);
        n = in.nextInt();
        m = in.nextInt();
        s = in.next().toCharArray();
        g = new int[m][m];
        for (int i = 1; i < n; i++) {
            int x = s[i - 1] - 'a', y = s[i] - 'a';
            if (x != y) {
                g[x][y]++;
                g[y][x]++;
            }
        }
        cost = new int[m][1 << m];
        for (int i = 0; i < m; i++) {
            int w = 0;
            for (int j = 0; j < m; j++) w += g[i][j];
            pre(i, 0, 0, -w);
        }
        dp = new int[1 << m];
        Arrays.fill(dp, -1);
        pw.println(solve(0, 0));

        pw.close();
    }

    static void pre(int x, int pos, int mask, int w) {
        if (pos >= m) {
            cost[x][mask] = w;
            return;
        }
        pre(x, pos + 1, mask, w);
        pre(x, pos + 1, set(mask, pos), w + 2 * g[x][pos]);
    }

    static int solve(int pos, int mask) {
        if (pos >= m) return 0;
        if (dp[mask] != -1) return dp[mask];
        int min = inf;
        for (int i = 0; i < m; i++) {
            if (!check(mask, i)) {
                int res = cost[i][mask] * pos + solve(pos + 1, set(mask, i));
                min = min(min, res);
            }
        }
        return dp[mask] = min;
    }

    static boolean check(int N, int pos) {
        return (N & (1 << pos)) != 0;
    }

    static int set(int N, int pos) {
        return N = N | (1 << pos);
    }

    static int reset(int N, int pos) {
        return N = N & ~(1 << pos);
    }

    static void debug(Object... obj) {
        System.err.println(Arrays.deepToString(obj));
    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream));
            tokenizer = null;
        }

        public String next() throws Exception {
            while (tokenizer == null || !tokenizer.hasMoreTokens())
                tokenizer = new StringTokenizer(reader.readLine());
            return tokenizer.nextToken();
        }

        public String nextLine() throws Exception {
            String line = null;
            tokenizer = null;
            line = reader.readLine();
            return line;
        }

        public int nextInt() throws Exception {
            return Integer.parseInt(next());
        }

        public double nextDouble() throws Exception {
            return Double.parseDouble(next());
        }

        public long nextLong() throws Exception {
            return Long.parseLong(next());
        }

    }
}


import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class E implements Runnable {
	public static void main (String[] args) {new Thread(null, new E(), "_cf", 1 << 28).start();}

	int n, m;
	char[] str;
	int[][] occs, cost;
	int[] dp;

	public void run() {
		FastScanner fs = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		System.err.println("");

		n = fs.nextInt(); m = fs.nextInt();
		byte[] str = fs.next().getBytes();
		int[] occs = new int[1<<m];
		for(int i = 0; i < n-1; i++) {
			int l1 = str[i] - 'a';
			int l2 = str[i+1] - 'a';
			occs[(1<<l1) | (1<<l2)]++;
			occs[(1<<l2) | (1<<l1)]++;
		}
		//cost[mask][v] = numPairs with v for some all bits on in mask
		int all = (1<<m)-1;
		cost = new int[m][1<<m];
		for(int i = 0; i < m; i++) {
			for(int mask = 1; mask < all; mask++) {
				if(((1<<i)&mask) > 0) continue;
				int lb = mask & (-mask);
				int trail = Integer.numberOfTrailingZeros(lb);
				int nmask = mask ^ lb;
				cost[i][mask] = cost[i][nmask]+occs[1<<i | 1<<trail];
			}
		}

		dp = new int[1<<m];
		for(int mask = dp.length-2; mask >= 0; mask--) {
			int addOn = 0;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				addOn += cost[nxt][mask];
			}
			int res = oo;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				int ret = addOn+dp[mask | (1<<nxt)];
				res = min(res, ret);
			}
			dp[mask] = res;
		}

		System.out.println(dp[0]>>1);

		out.close();
	}

	int oo = (int)1e9;
	int min(int a, int b) {
		if(a < b) return a;
		return b;
	}

	class FastScanner {
		public int BS = 1<<16;
		public char NC = (char)0;
		byte[] buf = new byte[BS];
		int bId = 0, size = 0;
		char c = NC;
		double num = 1;
		BufferedInputStream in;

		public FastScanner() {
			in = new BufferedInputStream(System.in, BS);
		}

		public FastScanner(String s) {
			try {
				in = new BufferedInputStream(new FileInputStream(new File(s)), BS);
			}
			catch (Exception e) {
				in = new BufferedInputStream(System.in, BS);
			}
		}

		public char nextChar(){
			while(bId==size) {
				try {
					size = in.read(buf);
				}catch(Exception e) {
					return NC;
				}
				if(size==-1)return NC;
				bId=0;
			}
			return (char)buf[bId++];
		}

		public int nextInt() {
			return (int)nextLong();
		}

		public long nextLong() {
			num=1;
			boolean neg = false;
			if(c==NC)c=nextChar();
			for(;(c<'0' || c>'9'); c = nextChar()) {
				if(c=='-')neg=true;
			}
			long res = 0;
			for(; c>='0' && c <='9'; c=nextChar()) {
				res = (res<<3)+(res<<1)+c-'0';
				num*=10;
			}
			return neg?-res:res;
		}

		public double nextDouble() {
			double cur = nextLong();
			return c!='.' ? cur:cur+nextLong()/num;
		}

		public String next() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c>32) {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public String nextLine() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c!='\n') {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public boolean hasNext() {
			if(c>32)return true;
			while(true) {
				c=nextChar();
				if(c==NC)return false;
				else if(c>32)return true;
			}
		}

		public int[] nextIntArray(int n) {
			int[] res = new int[n];
			for(int i = 0; i < n; i++) res[i] = nextInt();
			return res;
		}

	}

}


/**
 * @author egaeus
 * @mail sebegaeusprogram@gmail.com
 * @veredict Accepted
 * @url <https://codeforces.com/problemset/problem/1238/E>
 * @category dp
 * @date 19/10/2019
 **/

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.StringTokenizer;

import static java.lang.Integer.bitCount;
import static java.lang.Integer.parseInt;

public class CF1238E {
    static long[][] Q;
    static int N;
    static int M;
    static long[] mem;
    static long[] sums;

    public static void main(String args[]) throws Throwable {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        for (String ln; (ln = in.readLine()) != null; ) {
            StringTokenizer st = new StringTokenizer(ln);
            N = parseInt(st.nextToken());
            M = parseInt(st.nextToken());
            char[] S = in.readLine().toCharArray();
            Q = new long[M][M];
            mem = new long[1 << M];
            Arrays.fill(mem, -1);
            for (int i = 1; i < N; i++)
                Q[S[i - 1] - 'a'][S[i] - 'a'] = Q[S[i] - 'a'][S[i - 1] - 'a'] = Q[S[i - 1] - 'a'][S[i] - 'a'] + 1;
            calculateSums();
            for (int i = (1 << M) - 1; i >= 0; i--)
                f(i);
            System.out.println(f(0));
        }
    }

    static void calculateSums() {
        sums = new long[1 << M];
        Arrays.fill(sums, -1);
        for (int u = 0; u < (1 << M); u++) {
            if (sums[u] == -1) {
                sums[u] = 0;
                for (int j = 0; j < M; j++)
                    for (int k = j + 1; k < M; k++)
                        if (((u & (1 << j)) == 0 && (u & (1 << k)) != 0) ||
                                ((u & (1 << j)) != 0 && (u & (1 << k)) == 0))
                            sums[u] += Q[j][k];
                int neg = (~u) & ((1 << M) - 1);
                sums[neg] = sums[u];
            }
        }
    }

    static long f(int u) {
        if (bitCount(u) == M) return 0;
        if (mem[u] >= 0)
            return mem[u];
        long min = 10000000000L;
        for (int i = 0; i < M; i++)
            if ((u & (1 << i)) == 0) {
                u = u | (1 << i);
                min = Math.min(f(u) + sums[u], min);
                u = u ^ (1 << i);
            }
        return mem[u] = min;
    }
}


/*Author: Satyajeet Singh, Delhi Technological University*/
    import java.io.*;
    import java.util.*;
    import java.text.*;
    import java.lang.*;
    import java.math.*;

public class Main{
/*********************************************Constants******************************************/
    static PrintWriter out=new PrintWriter(new OutputStreamWriter(System.out));
    static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    static long mod=(long)1e9+7;
    static long mod1=998244353;
    static boolean sieve[];
    static ArrayList<Integer> primes;
    static long factorial[],invFactorial[];
    static ArrayList<Pair> graph[];
    static boolean oj = System.getProperty("ONLINE_JUDGE") != null;
/****************************************Solutions Begins***************************************/
    static int n,m;
    static long cost[][],dp[];
    public static void main(String[] args) throws Exception{
        String st[]=nl();
        n=pi(st[0]);
        m=pi(st[1]);
        st=nl();
        String str=st[0];
        int mn=10000;
        for(int i=0;i<n;i++){
            mn=Math.min(mn,str.charAt(i));
        }
        cost=new long[m][m];
        for(int i=1;i<n;i++){
            int a1=str.charAt(i-1)-mn;
            int a2=str.charAt(i)-mn;
            if(a1==a2)continue;
            cost[a1][a2]++;
            cost[a2][a1]++;
        }
        int mm=1<<m;
        dp=new long[mm];
        Arrays.fill(dp,Long.MAX_VALUE/2);
        dp[0]=0;
        //debug(cost);
        long cntbit[]=new long[mm];
        int minbit[]=new int[mm];

        for(int mask=1;mask<mm;mask++){
            cntbit[mask]=1+cntbit[(mask&(mask-1))];
            for(int i=0;i<m;i++){
                if(((mask>>i)&1)!=0){
                    minbit[mask]=i;
                    break;
                }
            }
        }
        long cntcost[][]=new long[mm][m];
        for(int mask=0;mask<mm;mask++){
            for(int i=0;i<m;i++){
                int b=minbit[mask];
                cntcost[mask][i]=cntcost[mask^(1<<b)][i]+cost[i][b];
            }
        }
        int yy=mm-1;
        for(int mask=0;mask<mm;mask++){
            long cnt=cntbit[mask];
            for(int i=0;i<m;i++){
                if(((mask>>i)&1)!=0){
                    long ans=cnt*(cntcost[mask][i]-cntcost[yy^mask][i]);
                    dp[mask]=Math.min(dp[mask],dp[mask^(1<<i)]+ans);
                }
            }
        }
        out.println(dp[mm-1]);
/****************************************Solutions Ends**************************************************/
        out.flush();
        out.close();
    }
/****************************************Template Begins************************************************/
    static String[] nl() throws Exception{
        return br.readLine().split(" ");
    }
    static String[] nls() throws Exception{
        return br.readLine().split("");
    }
    static int pi(String str) {
        return Integer.parseInt(str);
    }
    static long pl(String str){
        return Long.parseLong(str);
    }
    static double pd(String str){
        return Double.parseDouble(str);
    }
/***************************************Precision Printing**********************************************/
    static void printPrecision(double d){
        DecimalFormat ft = new DecimalFormat("0.000000000000000000000");
        out.println(ft.format(d));
    }
/**************************************Bit Manipulation**************************************************/
    static void printMask(long mask){
        System.out.println(Long.toBinaryString(mask));
    }
    static int countBit(long mask){
        int ans=0;
        while(mask!=0){
            if(mask%2==1){
                ans++;
            }
            mask/=2;
        }
        return ans;
    }
/******************************************Graph*********************************************************/
    static void Makegraph(int n){
        graph=new ArrayList[n];
        for(int i=0;i<n;i++){
            graph[i]=new ArrayList<>();
        }
    }
    static void addEdge(int a,int b){
        graph[a].add(new Pair(b,1));
    }
    static void addEdge(int a,int b,int c){
        graph[a].add(new Pair(b,c));
    }
/*********************************************PAIR********************************************************/
    static class PairComp implements Comparator<Pair>{
        public int compare(Pair p1,Pair p2){
            return p1.u-p2.u;
        }
    }
    static class Pair implements Comparable<Pair> {
        int u;
        int v;
        int index=-1;
        public Pair(int u, int v) {
            this.u = u;
            this.v = v;
        }
        public int hashCode() {
            int hu = (int) (u ^ (u >>> 32));
            int hv = (int) (v ^ (v >>> 32));
            return 31 * hu + hv;
        }
        public boolean equals(Object o) {
            Pair other = (Pair) o;
            return u == other.u && v == other.v;
        }
        public int compareTo(Pair other) {
            if(index!=other.index)
                return Long.compare(index, other.index);
            return Long.compare(v, other.v)!=0?Long.compare(v, other.v):Long.compare(u, other.u);
        }
        public String toString() {
            return "[u=" + u + ", v=" + v + "]";
        }
    }
/******************************************Long Pair*******************************************************/
    static class PairCompL implements Comparator<Pairl>{
        public int compare(Pairl p1,Pairl p2){
            long a=p1.u*p2.v;
            long b=p2.u*p1.v;
            if(a>b){
                return -1;
            }
            else if(a<b){
                return 1;
            }
            else{
                return 0;
            }
        }
    }
    static class Pairl implements Comparable<Pairl> {
            long u;
            long v;
            int index=-1;
            public Pairl(long u, long v) {
                this.u = u;
                this.v = v;
            }

            public int hashCode() {
                int hu = (int) (u ^ (u >>> 32));
                int hv = (int) (v ^ (v >>> 32));
                return 31 * hu + hv;
            }

            public boolean equals(Object o) {
                Pairl other = (Pairl) o;
                return u == other.u && v == other.v;
            }

            public int compareTo(Pairl other) {
                if(index!=other.index)
                    return Long.compare(index, other.index);
                return Long.compare(v, other.v)!=0?Long.compare(v, other.v):Long.compare(u, other.u);
            }

            public String toString() {
                return "[u=" + u + ", v=" + v + "]";
            }
        }
/*****************************************DEBUG***********************************************************/
    public static void debug(Object... o) {
        if(!oj)
        System.out.println(Arrays.deepToString(o));
    }
/************************************MODULAR EXPONENTIATION***********************************************/
    static long modulo(long a,long b,long c) {
        long x=1;
        long y=a%c;
        while(b > 0){
            if(b%2 == 1){
                x=(x*y)%c;
            }
            y = (y*y)%c; // squaring the base
            b /= 2;
        }
        return  x%c;
    }
/********************************************GCD**********************************************************/
    static long gcd(long x, long y)
    {
        if(x==0)
            return y;
        if(y==0)
            return x;
        long r=0, a, b;
        a = (x > y) ? x : y; // a is greater number
        b = (x < y) ? x : y; // b is smaller number
        r = b;
        while(a % b != 0)
        {
            r = a % b;
            a = b;
            b = r;
        }
        return r;
    }
/******************************************SIEVE**********************************************************/
    static void sieveMake(int n){
        sieve=new boolean[n];
        Arrays.fill(sieve,true);
        sieve[0]=false;
        sieve[1]=false;
        for(int i=2;i*i<n;i++){
            if(sieve[i]){
                for(int j=i*i;j<n;j+=i){
                    sieve[j]=false;
                }
            }
        }
        primes=new ArrayList<Integer>();
        for(int i=0;i<n;i++){
            if(sieve[i]){
                primes.add(i);
            }
        }
    }
/********************************************End***********************************************************/
}


import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.StringTokenizer;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());
        int n = Integer.parseInt(st.nextToken());
        int m = Integer.parseInt(st.nextToken());
        char[] arr = br.readLine().toCharArray();
        int[][] count = new int[m][m];
        for (int i = 0; i < n - 1; i++) {
            count[arr[i] - 'a'][arr[i + 1] - 'a']++;
            count[arr[i + 1] - 'a'][arr[i] - 'a']++;
        }
        int[] memo = new int[1 << m];
        Arrays.fill(memo, (int) 1e9);
        memo[0] = 0;
        for (int msk = 0; msk < 1 << m; msk++) {
            for (int c = 0; c < m; c++) {
                if ((msk & 1 << c) != 0)
                    continue;
                int temp = 0;
                for (int i = 0; i < m; i++) {
                    if (i == c)
                        continue;
                    if ((msk & 1 << i) != 0) {
                        temp += count[c][i];
                    } else
                        temp -= count[c][i];
                }
                memo[msk | 1 << c] = Math.min(memo[msk | 1 << c], temp*Integer.bitCount(msk) + memo[msk]);
            }
        }
        System.out.println(memo[(1 << m) - 1]);
    }
}


import java.util.*;
import java.io.*;

public class Main {
	public static void main(String args[]) {new Main().run();}

	FastReader in = new FastReader();
	PrintWriter out = new PrintWriter(System.out);
	void run(){
		out.println(work());
		out.flush();
	}
	long mod=1000000007;
	long gcd(long a,long b) {
		return b==0?a:gcd(b,a%b);
	}
	long work() {
		int n=in.nextInt();
		int m=in.nextInt();
		String str=in.next();
		long[] dp=new long[1<<m];
		long[][] cnt=new long[m][m];
		long[] rec=new long[1<<m];//记录每次移动的一位怎加的值，减少重复计算
		for(int i=1;i<n;i++) {
			int n1=str.charAt(i-1)-'a';
			int n2=str.charAt(i)-'a';
			cnt[n1][n2]++;
			cnt[n2][n1]++;
		}
		for(int i=1;i<1<<m;i++) {
			dp[i]=9999999999L;
			long v=0;
			int b=0;//最低位的1
			for(int j=0;j<m;j++) {
				if((i&(1<<j))>0) {
					b=j;
					break;
				}
			}
			for(int j=0;j<m;j++) {
				if((i&(1<<j))==0) {
					v+=cnt[b][j];
				}else {
					if(b!=j)v-=cnt[b][j];
				}
			}
			v+=rec[i-(1<<b)];
			for(int j=0;j<m;j++) {
				if((i&(1<<j))>0) {
					dp[i]=Math.min(dp[i], dp[i-(1<<j)]+v);
				}
			}
			rec[i]=v;
		}

		return dp[(1<<m)-1];
	}
}

class FastReader
{
	BufferedReader br;
	StringTokenizer st;

	public FastReader()
	{
		br=new BufferedReader(new InputStreamReader(System.in));
	}

	public String next()
	{
		if(st==null || !st.hasMoreElements())
		{
			try {
				st = new StringTokenizer(br.readLine());
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
		return st.nextToken();
	}

	public int nextInt()
	{
		return Integer.parseInt(next());
	}

	public long nextLong()
	{
		return Long.parseLong(next());
	}
}


/**
 * @author Juan Sebastian Beltran Rojas
 * @mail jsbeltran.valhalla@gmail.com
 * @veredict
 * @url https://codeforces.com/problemset/problem/1238/E
 * @category dp
 * @date 19/10/2019
 **/

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.StringTokenizer;

import static java.lang.Integer.bitCount;
import static java.lang.Integer.parseInt;

public class CF1238E {
    static long[][] Q;
    static int N;
    static int M;
    static long[] mem;
    static long[] sums;

    public static void main(String args[]) throws Throwable {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        for (String ln; (ln = in.readLine()) != null; ) {
            StringTokenizer st = new StringTokenizer(ln);
            N = parseInt(st.nextToken());
            M = parseInt(st.nextToken());
            char[] S = in.readLine().toCharArray();
            Q = new long[M][M];
            mem = new long[1 << M];
            Arrays.fill(mem, -1);
            for (int i = 1; i < N; i++)
                Q[S[i - 1] - 'a'][S[i] - 'a'] = Q[S[i] - 'a'][S[i - 1] - 'a'] = Q[S[i - 1] - 'a'][S[i] - 'a'] + 1;
            calculateSums();
            for (int i = (1 << M) - 1; i >= 0; i--)
                f(i);
            System.out.println(f(0));
        }
    }

    static void calculateSums() {
        sums = new long[1 << M];
        Arrays.fill(sums, -1);
        for (int u = 0; u < (1 << M); u++) {
            if (sums[u] == -1) {
                sums[u] = 0;
                for (int j = 0; j < M; j++)
                    for (int k = j + 1; k < M; k++)
                        if (((u & (1 << j)) == 0 && (u & (1 << k)) != 0) ||
                                ((u & (1 << j)) != 0 && (u & (1 << k)) == 0))
                            sums[u] += Q[j][k];
                int neg = (~u) & ((1 << M) - 1);
                sums[neg] = sums[u];
            }
        }
    }

    static long f(int u) {
        if (bitCount(u) == M) return 0;
        if (mem[u] >= 0)
            return mem[u];
        long min = 10000000000L;
        for (int i = 0; i < M; i++)
            if ((u & (1 << i)) == 0) {
                u = u | (1 << i);
                min = Math.min(f(u) + sums[u], min);
                u = u ^ (1 << i);
            }
        return mem[u] = min;
    }
}


import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class E implements Runnable {
	public static void main (String[] args) {new Thread(null, new E(), "_cf", 1 << 28).start();}

	int n, m;
	char[] str;
	int[][] occs, cost;
	int[] dp;

	public void run() {
		FastScanner fs = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		System.err.println("");

		n = fs.nextInt(); m = fs.nextInt();
		str = fs.next().toCharArray();
		occs = new int[m][m];
		for(int i = 0; i < n-1; i++) {
			occs[str[i]-'a'][str[i+1]-'a']++;
			occs[str[i+1]-'a'][str[i]-'a']++;
		}
		//cost[mask][v] = numPairs with v for some all bits on in mask
		int all = (1<<m)-1;
		cost = new int[m][1<<m];
		for(int i = 0; i < m; i++) {
			for(int mask = 1; mask < all; mask++) {
				if(((1<<i)&mask) > 0) continue;
				int lb = mask & (-mask);
				int trail = Integer.numberOfTrailingZeros(lb);
				int nmask = mask ^ lb;
				cost[i][mask] = cost[i][nmask]+occs[i][trail];
			}
		}

		dp = new int[1<<m];
		for(int mask = dp.length-2; mask >= 0; mask--) {
			int addOn = 0;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				addOn += cost[nxt][mask];
			}
			int res = oo;
			for(int nxt = 0; nxt < m; nxt++) {
				if(((1<<nxt)&mask) > 0) continue;
				int ret = addOn+dp[mask | (1<<nxt)];
				res = min(res, ret);
			}
			dp[mask] = res;
		}

		System.out.println(dp[0]);

		out.close();
	}

	int oo = (int)1e9;
	int min(int a, int b) {
		if(a < b) return a;
		return b;
	}

	class FastScanner {
		public int BS = 1<<16;
		public char NC = (char)0;
		byte[] buf = new byte[BS];
		int bId = 0, size = 0;
		char c = NC;
		double num = 1;
		BufferedInputStream in;

		public FastScanner() {
			in = new BufferedInputStream(System.in, BS);
		}

		public FastScanner(String s) {
			try {
				in = new BufferedInputStream(new FileInputStream(new File(s)), BS);
			}
			catch (Exception e) {
				in = new BufferedInputStream(System.in, BS);
			}
		}

		public char nextChar(){
			while(bId==size) {
				try {
					size = in.read(buf);
				}catch(Exception e) {
					return NC;
				}
				if(size==-1)return NC;
				bId=0;
			}
			return (char)buf[bId++];
		}

		public int nextInt() {
			return (int)nextLong();
		}

		public long nextLong() {
			num=1;
			boolean neg = false;
			if(c==NC)c=nextChar();
			for(;(c<'0' || c>'9'); c = nextChar()) {
				if(c=='-')neg=true;
			}
			long res = 0;
			for(; c>='0' && c <='9'; c=nextChar()) {
				res = (res<<3)+(res<<1)+c-'0';
				num*=10;
			}
			return neg?-res:res;
		}

		public double nextDouble() {
			double cur = nextLong();
			return c!='.' ? cur:cur+nextLong()/num;
		}

		public String next() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c>32) {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public String nextLine() {
			StringBuilder res = new StringBuilder();
			while(c<=32)c=nextChar();
			while(c!='\n') {
				res.append(c);
				c=nextChar();
			}
			return res.toString();
		}

		public boolean hasNext() {
			if(c>32)return true;
			while(true) {
				c=nextChar();
				if(c==NC)return false;
				else if(c>32)return true;
			}
		}

		public int[] nextIntArray(int n) {
			int[] res = new int[n];
			for(int i = 0; i < n; i++) res[i] = nextInt();
			return res;
		}

	}

}


import java.io.*;
import java.util.*;

import static java.lang.Math.*;
import static java.util.Arrays.*;

// editorial sol

public class cf1238e {

    public static void main(String[] args) throws IOException {
        int n = rni(), m = ni(), cnt[][] = new int[m][m], dp[] = new int[1 << m], all = (1 << m) - 1;
        char[] s = rcha();
        for (int i = 1; i < n; ++i) {
            ++cnt[s[i] - 'a'][s[i - 1] - 'a'];
            ++cnt[s[i - 1] - 'a'][s[i] - 'a'];
        }
        fill(dp, IBIG);
        dp[0] = 0;
        int cnt_bit[] = new int[1 << m], min_bit[] = new int[1 << m], d[][] = new int[1 << m][m];
        for (int mask = 1; mask <= all; ++mask) {
            cnt_bit[mask] = 1 + cnt_bit[mask & (mask - 1)];
            for (int i = 0; i < n; ++i) {
                if ((mask & (1 << i)) > 0) {
                    min_bit[mask] = i;
                    break;
                }
            }
        }
        for (int mask = 1; mask <= all; ++mask) {
            for (int i = 0; i < m; ++i) {
                d[mask][i] = d[mask ^ (1 << min_bit[mask])][i] + cnt[i][min_bit[mask]];
            }
        }
        for (int mask = 0; mask <= all; ++mask) {
            for (int i = 0; i < m; ++i) {
                if ((mask & (1 << i)) > 0) {
                    continue;
                }
                int pos = cnt_bit[mask], next = mask | (1 << i);
                dp[next] = min(dp[next], dp[mask] + pos * (d[mask][i] - d[all ^ next][i]));
            }
        }
        prln(dp[all]);
        close();
    }

    static BufferedReader __in = new BufferedReader(new InputStreamReader(System.in));
    static PrintWriter __out = new PrintWriter(new OutputStreamWriter(System.out));
    static StringTokenizer input;
    static Random __rand = new Random();

    // references
    // IBIG = 1e9 + 7
    // IMAX ~= 2e9
    // LMAX ~= 9e18

    // constants
    static final int IBIG = 1000000007;
    static final int IMAX = 2147483647;
    static final int IMIN = -2147483648;
    static final long LMAX = 9223372036854775807L;
    static final long LMIN = -9223372036854775808L;
    // math util
    static int minof(int a, int b, int c) {return min(a, min(b, c));}
    static int minof(int... x) {if (x.length == 1) return x[0]; if (x.length == 2) return min(x[0], x[1]); if (x.length == 3) return min(x[0], min(x[1], x[2])); int min = x[0]; for (int i = 1; i < x.length; ++i) if (x[i] < min) min = x[i]; return min;}
    static long minof(long a, long b, long c) {return min(a, min(b, c));}
    static long minof(long... x) {if (x.length == 1) return x[0]; if (x.length == 2) return min(x[0], x[1]); if (x.length == 3) return min(x[0], min(x[1], x[2])); long min = x[0]; for (int i = 1; i < x.length; ++i) if (x[i] < min) min = x[i]; return min;}
    static int maxof(int a, int b, int c) {return max(a, max(b, c));}
    static int maxof(int... x) {if (x.length == 1) return x[0]; if (x.length == 2) return max(x[0], x[1]); if (x.length == 3) return max(x[0], max(x[1], x[2])); int max = x[0]; for (int i = 1; i < x.length; ++i) if (x[i] > max) max = x[i]; return max;}
    static long maxof(long a, long b, long c) {return max(a, max(b, c));}
    static long maxof(long... x) {if (x.length == 1) return x[0]; if (x.length == 2) return max(x[0], x[1]); if (x.length == 3) return max(x[0], max(x[1], x[2])); long max = x[0]; for (int i = 1; i < x.length; ++i) if (x[i] > max) max = x[i]; return max;}
    static int powi(int a, int b) {if (a == 0) return 0; int ans = 1; while (b > 0) {if ((b & 1) > 0) ans *= a; a *= a; b >>= 1;} return ans;}
    static long powl(long a, int b) {if (a == 0) return 0; long ans = 1; while (b > 0) {if ((b & 1) > 0) ans *= a; a *= a; b >>= 1;} return ans;}
    static int fli(double d) {return (int) d;}
    static int cei(double d) {return (int) ceil(d);}
    static long fll(double d) {return (long) d;}
    static long cel(double d) {return (long) ceil(d);}
    static int gcf(int a, int b) {return b == 0 ? a : gcf(b, a % b);}
    static long gcf(long a, long b) {return b == 0 ? a : gcf(b, a % b);}
    static int lcm(int a, int b) {return a * b / gcf(a, b);}
    static long lcm(long a, long b) {return a * b / gcf(a, b);}
    static int randInt(int min, int max) {return __rand.nextInt(max - min + 1) + min;}
    static long mix(long x) {x += 0x9e3779b97f4a7c15L; x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9L; x = (x ^ (x >> 27)) * 0x94d049bb133111ebL; return x ^ (x >> 31);}
    // array util
    static void reverse(int[] a) {for (int i = 0, n = a.length, half = n / 2; i < half; ++i) {int swap = a[i]; a[i] = a[n - i - 1]; a[n - i - 1] = swap;}}
    static void reverse(long[] a) {for (int i = 0, n = a.length, half = n / 2; i < half; ++i) {long swap = a[i]; a[i] = a[n - i - 1]; a[n - i - 1] = swap;}}
    static void reverse(double[] a) {for (int i = 0, n = a.length, half = n / 2; i < half; ++i) {double swap = a[i]; a[i] = a[n - i - 1]; a[n - i - 1] = swap;}}
    static void reverse(char[] a) {for (int i = 0, n = a.length, half = n / 2; i < half; ++i) {char swap = a[i]; a[i] = a[n - i - 1]; a[n - i - 1] = swap;}}
    static void shuffle(int[] a) {int n = a.length - 1; for (int i = 0; i < n; ++i) {int ind = randInt(i, n); int swap = a[i]; a[i] = a[ind]; a[ind] = swap;}}
    static void shuffle(long[] a) {int n = a.length - 1; for (int i = 0; i < n; ++i) {int ind = randInt(i, n); long swap = a[i]; a[i] = a[ind]; a[ind] = swap;}}
    static void shuffle(double[] a) {int n = a.length - 1; for (int i = 0; i < n; ++i) {int ind = randInt(i, n); double swap = a[i]; a[i] = a[ind]; a[ind] = swap;}}
    static void rsort(int[] a) {shuffle(a); sort(a);}
    static void rsort(long[] a) {shuffle(a); sort(a);}
    static void rsort(double[] a) {shuffle(a); sort(a);}
    static int[] copy(int[] a) {int[] ans = new int[a.length]; for (int i = 0; i < a.length; ++i) ans[i] = a[i]; return ans;}
    static long[] copy(long[] a) {long[] ans = new long[a.length]; for (int i = 0; i < a.length; ++i) ans[i] = a[i]; return ans;}
    static double[] copy(double[] a) {double[] ans = new double[a.length]; for (int i = 0; i < a.length; ++i) ans[i] = a[i]; return ans;}
    static char[] copy(char[] a) {char[] ans = new char[a.length]; for (int i = 0; i < a.length; ++i) ans[i] = a[i]; return ans;}
    // graph util
    static List<List<Integer>> g(int n) {List<List<Integer>> g = new ArrayList<>(); for (int i = 0; i < n; ++i) g.add(new ArrayList<>()); return g;}
    static List<Set<Integer>> sg(int n) {List<Set<Integer>> g = new ArrayList<>(); for (int i = 0; i < n; ++i) g.add(new HashSet<>()); return g;}
    static void c(List<? extends Collection<Integer>> g, int u, int v) {g.get(u).add(v); g.get(v).add(u);}
    static void cto(List<? extends Collection<Integer>> g, int u, int v) {g.get(u).add(v);}
    static void dc(List<? extends Collection<Integer>> g, int u, int v) {g.get(u).remove(v); g.get(v).remove(u);}
    static void dcto(List<? extends Collection<Integer>> g, int u, int v) {g.get(u).remove(v);}
    // input
    static void r() throws IOException {input = new StringTokenizer(rline());}
    static int ri() throws IOException {return Integer.parseInt(rline());}
    static long rl() throws IOException {return Long.parseLong(rline());}
    static double rd() throws IOException {return Double.parseDouble(rline());}
    static int[] ria(int n) throws IOException {int[] a = new int[n]; r(); for (int i = 0; i < n; ++i) a[i] = ni(); return a;}
    static int[] riam1(int n) throws IOException {int[] a = new int[n]; r(); for (int i = 0; i < n; ++i) a[i] = ni() - 1; return a;}
    static long[] rla(int n) throws IOException {long[] a = new long[n]; r(); for (int i = 0; i < n; ++i) a[i] = nl(); return a;}
    static double[] rda(int n) throws IOException {double[] a = new double[n]; r(); for (int i = 0; i < n; ++i) a[i] = nd(); return a;}
    static char[] rcha() throws IOException {return rline().toCharArray();}
    static String rline() throws IOException {return __in.readLine();}
    static String n() {return input.nextToken();}
    static int rni() throws IOException {r(); return ni();}
    static int ni() {return Integer.parseInt(n());}
    static long rnl() throws IOException {r(); return nl();}
    static long nl() {return Long.parseLong(n());}
    static double rnd() throws IOException {r(); return nd();}
    static double nd() {return Double.parseDouble(n());}
    static List<List<Integer>> rg(int n, int m) throws IOException {List<List<Integer>> g = g(n); for (int i = 0; i < m; ++i) c(g, rni() - 1, ni() - 1); return g;}
    static void rg(List<List<Integer>> g, int m) throws IOException {for (int i = 0; i < m; ++i) c(g, rni() - 1, ni() - 1);}
    static List<List<Integer>> rdg(int n, int m) throws IOException {List<List<Integer>> g = g(n); for (int i = 0; i < m; ++i) cto(g, rni() - 1, ni() - 1); return g;}
    static void rdg(List<List<Integer>> g, int m) throws IOException {for (int i = 0; i < m; ++i) cto(g, rni() - 1, ni() - 1);}
    static List<Set<Integer>> rsg(int n, int m) throws IOException {List<Set<Integer>> g = sg(n); for (int i = 0; i < m; ++i) c(g, rni() - 1, ni() - 1); return g;}
    static void rsg(List<Set<Integer>> g, int m) throws IOException {for (int i = 0; i < m; ++i) c(g, rni() - 1, ni() - 1);}
    static List<Set<Integer>> rdsg(int n, int m) throws IOException {List<Set<Integer>> g = sg(n); for (int i = 0; i < m; ++i) cto(g, rni() - 1, ni() - 1); return g;}
    static void rdsg(List<Set<Integer>> g, int m) throws IOException {for (int i = 0; i < m; ++i) cto(g, rni() - 1, ni() - 1);}
    // output
    static void pr(int i) {__out.print(i);}
    static void prln(int i) {__out.println(i);}
    static void pr(long l) {__out.print(l);}
    static void prln(long l) {__out.println(l);}
    static void pr(double d) {__out.print(d);}
    static void prln(double d) {__out.println(d);}
    static void pr(char c) {__out.print(c);}
    static void prln(char c) {__out.println(c);}
    static void pr(char[] s) {__out.print(new String(s));}
    static void prln(char[] s) {__out.println(new String(s));}
    static void pr(String s) {__out.print(s);}
    static void prln(String s) {__out.println(s);}
    static void pr(Object o) {__out.print(o);}
    static void prln(Object o) {__out.println(o);}
    static void prln() {__out.println();}
    static void pryes() {prln("yes");}
    static void pry() {prln("Yes");}
    static void prY() {prln("YES");}
    static void prno() {prln("no");}
    static void prn() {prln("No");}
    static void prN() {prln("NO");}
    static void pryesno(boolean b) {prln(b ? "yes" : "no");};
    static void pryn(boolean b) {prln(b ? "Yes" : "No");}
    static void prYN(boolean b) {prln(b ? "YES" : "NO");}
    static void prln(int... a) {for (int i = 0, len = a.length - 1; i < len; pr(a[i]), pr(' '), ++i); if (a.length > 0) prln(a[a.length - 1]); else prln();}
    static void prln(long... a) {for (int i = 0, len = a.length - 1; i < len; pr(a[i]), pr(' '), ++i); if (a.length > 0) prln(a[a.length - 1]); else prln();}
    static void prln(double... a) {for (int i = 0, len = a.length - 1; i < len; pr(a[i]), pr(' '), ++i); if (a.length > 0) prln(a[a.length - 1]); else prln();}
    static <T> void prln(Collection<T> c) {int n = c.size() - 1; Iterator<T> iter = c.iterator(); for (int i = 0; i < n; pr(iter.next()), pr(' '), ++i); if (n >= 0) prln(iter.next()); else prln();}
    static void h() {prln("hlfd"); flush();}
    static void flush() {__out.flush();}
    static void close() {__out.close();}}