#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, m;
  cin >> n >> m;
  string s;
  cin >> s;
  int c[20][20];
  int d[(1 << 20)];
  int an = 0;
  for (long long i = 0; i < n; i++) {
    an = an | (1 << (s[i] - 'a'));
  }
  for (long long i = 0; i < 20; i++) {
    for (long long j = 0; j < 20; j++) {
      c[i][j] = 0;
    }
  }
  for (long long i = 0; i < (1 << 20); i++) d[i] = 1000000000;
  for (long long i = 0; i < n - 1; i++) {
    if (s[i] != s[i + 1]) {
      c[s[i] - 'a'][s[i + 1] - 'a']++;
      c[s[i + 1] - 'a'][s[i] - 'a']++;
    }
  }
  int ct = 0;
  d[0] = 0;
  for (long long i = 1; i < (1 << 20); i++) {
    for (long long j = 0; j < 20; j++) {
      if (((1 << j) & i) > 0) {
        ct = 0;
        for (long long k = 0; k < 20; k++) {
          if (((1 << k) & i) > 0) {
            ct += c[j][k] * __builtin_popcount(i);
          } else {
            ct -= c[j][k] * __builtin_popcount(i);
          }
        }
        d[i] = min(d[i], d[i ^ (1 << j)] + ct);
      }
    }
  }
  cout << d[an] << "\n";
  return 0;
}
