source_code: import java.util.*;
  import java.io.*;
  
  public class E {
      public static void main(String[] args) {
          FastScanner scanner = new FastScanner();
          PrintWriter out = new PrintWriter(System.out, false);
          int n = scanner.nextInt();
          int m = scanner.nextInt();
          char[] str = scanner.next().toCharArray();
          int maxMask = 1 << m;
          long[] dp = new long[maxMask];
          int[][] dists = new int[m][m];
          for(int i = 1; i < n; i++) {
              int c1 = str[i] - 'a';
              int c2 = str[i-1] - 'a';
              dists[c1][c2]++;
              dists[c2][c1]++;
          }
          int[] pre = new int[maxMask];
          for(int mask = 0; mask < maxMask; mask++) {
              for(int i = 0; i < m; i++) {
                  if (((1 << i) & mask) == 0) continue;
                  for(int j = 0; j < m; j++) {
                      if (((1 << j) & mask) > 0) continue;
                      pre[mask] += dists[i][j];
                  }
              }
          }
          Arrays.fill(dp, Long.MAX_VALUE/4);
          dp[0] = 0;
          for(int mask = 0; mask < maxMask; mask++) {
              if (dp[mask] == Long.MAX_VALUE/4) continue;
              for(int i = 0; i < m; i++) {
                  if (((1 << i) & mask) > 0) continue;
                  int nmask = mask | (1 << i);
                  dp[nmask] = Math.min(dp[nmask], dp[mask] + pre[nmask]);
              }
          }
          out.println(dp[maxMask - 1]);
          out.flush();
      }
      
      public static class FastScanner {
          BufferedReader br;
          StringTokenizer st;
          
          public FastScanner(Reader in) {
              br = new BufferedReader(in);
          }
          
          public FastScanner() {
              this(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 readNextLine() {
              String str = "";
              try {
                  str = br.readLine();
              } catch (IOException e) {
                  e.printStackTrace();
              }
              return str;
          }
      }
  }
  