// Problem: 1238_E. Keyboard Purchase
// Time complexity: np
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.io.FilterInputStream;
import java.io.BufferedInputStream;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Jenish
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        ScanReader in = new ScanReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        EKeyboardPurchase solver = new EKeyboardPurchase();
        solver.solve(1, in, out);
        out.close();
    }

    static class EKeyboardPurchase {
        public void solve(int testNumber, ScanReader in, PrintWriter out) {
            int n = in.scanInt();
            int m = in.scanInt();
            int step[][] = new int[m][m];
            char arr[] = in.scanString().toCharArray();
            for (int i = 0; i < n - 1; i++) {
                step[arr[i] - 'a'][arr[i + 1] - 'a']++;
                step[arr[i + 1] - 'a'][arr[i] - 'a']++;
            }


            int dp[] = new int[1 << m];
            Arrays.fill(dp, Integer.MAX_VALUE / 2);
            for (int i = 0; i < m; i++) dp[1 << i] = 0;
            for (int i = 0; i < (1 << m); i++) {
                int cost = 0;
                for (int j = 0; j < m; j++) {
                    if (((i & (1 << j)) != 0)) {
                        for (int k = 0; k < m; k++) {
                            if (((i & (1 << k)) == 0)) {
                                cost += step[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] + cost);


            }


            out.println(dp[(1 << m) - 1]);

        }

    }

    static class ScanReader {
        private byte[] buf = new byte[4 * 1024];
        private int INDEX;
        private BufferedInputStream in;
        private int TOTAL;

        public ScanReader(InputStream inputStream) {
            in = new BufferedInputStream(inputStream);
        }

        private int scan() {
            if (INDEX >= TOTAL) {
                INDEX = 0;
                try {
                    TOTAL = in.read(buf);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                if (TOTAL <= 0) return -1;
            }
            return buf[INDEX++];
        }

        public int scanInt() {
            int I = 0;
            int n = scan();
            while (isWhiteSpace(n)) n = scan();
            int neg = 1;
            if (n == '-') {
                neg = -1;
                n = scan();
            }
            while (!isWhiteSpace(n)) {
                if (n >= '0' && n <= '9') {
                    I *= 10;
                    I += n - '0';
                    n = scan();
                }
            }
            return neg * I;
        }

        public String scanString() {
            int c = scan();
            while (isWhiteSpace(c)) c = scan();
            StringBuilder RESULT = new StringBuilder();
            do {
                RESULT.appendCodePoint(c);
                c = scan();
            } while (!isWhiteSpace(c));
            return RESULT.toString();
        }

        private boolean isWhiteSpace(int n) {
            if (n == ' ' || n == '\n' || n == '\r' || n == '\t' || n == -1) return true;
            else return false;
        }

    }
}