// Problem: 1238_E. Keyboard Purchase
// Time complexity: np


import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.charset.Charset;
import java.util.Arrays;

public class CF1238E {
    public static void main(String[] args) throws Exception {
        boolean local = System.getSecurityManager() == null;
        boolean async = false;

        Charset charset = Charset.forName("ascii");

        FastIO io = local ? new FastIO(new FileInputStream("D:\\DATABASE\\TESTCASE\\Code.in"), System.out, charset) : new FastIO(System.in, System.out, charset);
        Task task = new Task(io, new Debug(local));

        if (async) {
            Thread t = new Thread(null, task, "dalt", 1 << 27);
            t.setPriority(Thread.MAX_PRIORITY);
            t.start();
            t.join();
        } else {
            task.run();
        }

        if (local) {
            io.cache.append("\n\n--memory -- \n" + ((Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory()) >> 20) + "M");
        }

        io.flush();
    }

    public static class Task implements Runnable {
        final FastIO io;
        final Debug debug;
        int inf = (int) 1e8;
        long lInf = (long) 1e18;
        double dInf = 1e50;

        public Task(FastIO io, Debug debug) {
            this.io = io;
            this.debug = debug;
        }

        @Override
        public void run() {
            solve();
        }


        int[][] fee;
        int m;
        long[] dp;
        int mask;
        int[][] maskFee;

        public void solve() {
            int n = io.readInt();
            m = io.readInt();
            char[] s = new char[n];
            io.readString(s, 0);
            for (int i = 0; i < n; i++) {
                s[i] -= 'a';
            }
            fee = new int[m][m];
            for (int i = 1; i < n; i++) {
                fee[s[i]][s[i - 1]]++;
                fee[s[i - 1]][s[i]]++;
            }

            mask = (1 << m) - 1;
            maskFee = new int[m][mask + 1];
            SubsetGenerator sg = new SubsetGenerator();
            for (int i = 0; i < m; i++) {
                for (int j = 0; j < m; j++) {
                    sg.meanings[j] = fee[i][j];
                }
                for (int j = 0; j <= mask; j++) {
                    maskFee[i][j] = sg.next();
                }
            }

            dp = new long[mask + 1];
            Arrays.fill(dp, -1);
            dp[0] = 0;

            io.cache.append(dp(mask));
        }

        BitOperator bo = new BitOperator();

        public long dp(int s) {
            if (dp[s] == -1) {
                long extra = 0;
                int remainSet = mask - s;
                for (int j = 0; j < m; j++) {
                    if (bo.bitAt(s, j) == 0) {
                        continue;
                    }
                    extra += maskFee[j][remainSet];
                }

                dp[s] = lInf;
                for (int j = 0; j < m; j++) {
                    if (bo.bitAt(s, j) == 0) {
                        continue;
                    }
                    int ss = bo.setBit(s, j, false);
                    dp[s] = Math.min(dp[s], extra + dp(ss));
                }
            }

            return dp[s];
        }
    }

    /**
     * Bit operations
     */
    public static class BitOperator {
        public int bitAt(int x, int i) {
            return (x >> i) & 1;
        }

        public int bitAt(long x, int i) {
            return (int) ((x >> i) & 1);
        }

        public int setBit(int x, int i, boolean v) {
            if (v) {
                x |= 1 << i;
            } else {
                x &= ~(1 << i);
            }
            return x;
        }

        public long setBit(long x, int i, boolean v) {
            if (v) {
                x |= 1L << i;
            } else {
                x &= ~(1L << i);
            }
            return x;
        }

        public long swapBit(long x, int i, int j) {
            int bi = bitAt(x, i);
            int bj = bitAt(x, j);
            x = setBit(x, i, bj == 1);
            x = setBit(x, j, bi == 1);
            return x;
        }

        public int swapBit(int x, int i, int j) {
            int bi = bitAt(x, i);
            int bj = bitAt(x, j);
            x = setBit(x, i, bj == 1);
            x = setBit(x, j, bi == 1);
            return x;
        }

        /**
         * Determine whether x is subset of y
         */
        public boolean subset(long x, long y) {
            return intersect(x, y) == x;
        }

        /**
         * Merge two set
         */
        public long merge(long x, long y) {
            return x | y;
        }

        public long intersect(long x, long y) {
            return x & y;
        }

        public long differ(long x, long y) {
            return x - intersect(x, y);
        }
    }

    public static class SubsetGenerator {
        private int[] meanings = new int[33];
        private int[] bits = new int[33];
        private int remain;
        private int next;

        public void setSet(int set) {
            int bitCount = 0;
            while (set != 0) {
                meanings[bitCount] = set & -set;
                bits[bitCount] = 0;
                set -= meanings[bitCount];
                bitCount++;
            }
            remain = 1 << bitCount;
            next = 0;
        }

        public boolean hasNext() {
            return remain > 0;
        }

        private void consume() {
            remain = remain - 1;
            int i;
            for (i = 0; bits[i] == 1; i++) {
                bits[i] = 0;
                next -= meanings[i];
            }
            bits[i] = 1;
            next += meanings[i];
        }

        public int next() {
            int returned = next;
            consume();
            return returned;
        }
    }

    public static class FastIO {
        public final StringBuilder cache = new StringBuilder(1 << 13);
        private final InputStream is;
        private final OutputStream os;
        private final Charset charset;
        private StringBuilder defaultStringBuf = new StringBuilder(1 << 13);
        private byte[] buf = new byte[1 << 13];
        private int bufLen;
        private int bufOffset;
        private int next;

        public FastIO(InputStream is, OutputStream os, Charset charset) {
            this.is = is;
            this.os = os;
            this.charset = charset;
        }

        public FastIO(InputStream is, OutputStream os) {
            this(is, os, Charset.forName("ascii"));
        }

        private int read() {
            while (bufLen == bufOffset) {
                bufOffset = 0;
                try {
                    bufLen = is.read(buf);
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
                if (bufLen == -1) {
                    return -1;
                }
            }
            return buf[bufOffset++];
        }

        public void skipBlank() {
            while (next >= 0 && next <= 32) {
                next = read();
            }
        }

        public int readInt() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            int val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

        public long readLong() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            long val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

        public double readDouble() {
            boolean sign = true;
            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+';
                next = read();
            }

            long val = 0;
            while (next >= '0' && next <= '9') {
                val = val * 10 + next - '0';
                next = read();
            }
            if (next != '.') {
                return sign ? val : -val;
            }
            next = read();
            long radix = 1;
            long point = 0;
            while (next >= '0' && next <= '9') {
                point = point * 10 + next - '0';
                radix = radix * 10;
                next = read();
            }
            double result = val + (double) point / radix;
            return sign ? result : -result;
        }

        public String readString(StringBuilder builder) {
            skipBlank();

            while (next > 32) {
                builder.append((char) next);
                next = read();
            }

            return builder.toString();
        }

        public String readString() {
            defaultStringBuf.setLength(0);
            return readString(defaultStringBuf);
        }

        public int readLine(char[] data, int offset) {
            int originalOffset = offset;
            while (next != -1 && next != '\n') {
                data[offset++] = (char) next;
                next = read();
            }
            return offset - originalOffset;
        }

        public int readString(char[] data, int offset) {
            skipBlank();

            int originalOffset = offset;
            while (next > 32) {
                data[offset++] = (char) next;
                next = read();
            }

            return offset - originalOffset;
        }

        public int readString(byte[] data, int offset) {
            skipBlank();

            int originalOffset = offset;
            while (next > 32) {
                data[offset++] = (byte) next;
                next = read();
            }

            return offset - originalOffset;
        }

        public char readChar() {
            skipBlank();
            char c = (char) next;
            next = read();
            return c;
        }

        public void flush() throws IOException {
            os.write(cache.toString().getBytes(charset));
            os.flush();
            cache.setLength(0);
        }

        public boolean hasMore() {
            skipBlank();
            return next != -1;
        }
    }

    public static class Debug {
        private boolean allowDebug;

        public Debug(boolean allowDebug) {
            this.allowDebug = allowDebug;
        }

        public void assertTrue(boolean flag) {
            if (!allowDebug) {
                return;
            }
            if (!flag) {
                fail();
            }
        }

        public void fail() {
            throw new RuntimeException();
        }

        public void assertFalse(boolean flag) {
            if (!allowDebug) {
                return;
            }
            if (flag) {
                fail();
            }
        }

        private void outputName(String name) {
            System.out.print(name + " = ");
        }

        public void debug(String name, int x) {
            if (!allowDebug) {
                return;
            }

            outputName(name);
            System.out.println("" + x);
        }

        public void debug(String name, long x) {
            if (!allowDebug) {
                return;
            }
            outputName(name);
            System.out.println("" + x);
        }

        public void debug(String name, double x) {
            if (!allowDebug) {
                return;
            }
            outputName(name);
            System.out.println("" + x);
        }

        public void debug(String name, int[] x) {
            if (!allowDebug) {
                return;
            }
            outputName(name);
            System.out.println(Arrays.toString(x));
        }

        public void debug(String name, long[] x) {
            if (!allowDebug) {
                return;
            }
            outputName(name);
            System.out.println(Arrays.toString(x));
        }

        public void debug(String name, double[] x) {
            if (!allowDebug) {
                return;
            }
            outputName(name);
            System.out.println(Arrays.toString(x));
        }

        public void debug(String name, Object x) {
            if (!allowDebug) {
                return;
            }
            outputName(name);
            System.out.println("" + x);
        }

        public void debug(String name, Object... x) {
            if (!allowDebug) {
                return;
            }
            outputName(name);
            System.out.println(Arrays.deepToString(x));
        }
    }
}