source_code: 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;
          }
  
      }
  }
  
  