#include <bits/stdc++.h>
using namespace std;
int f[(1 << 20)], cnt[(1 << 20)], n, m;
string s;
int main() {
  cin >> n >> m >> s;
  fill(f + 1, f + (1 << m), INT_MAX);
  for (int i = 1; i < n; i++) {
    int a = s[i - 1] - 'a', b = s[i] - 'a';
    if (a != b) cnt[(1 << a) | (1 << b)]++;
  }
  for (int i = 0; i < (1 << m); i++) {
    int c = m - __builtin_popcount(i);
    for (int j = 0; j < m; j++) {
      if ((i & (1 << j)) == 0) {
        int num = f[i];
        for (int k = 0; k < m; k++) {
          int flag = 1;
          if (i & (1 << k)) flag = -1;
          num += flag * cnt[(1 << k) | (1 << j)] * c;
        }
        f[i | (1 << j)] = min(f[i | (1 << j)], num);
      }
    }
  }
  cout << f[(1 << m) - 1] << endl;
  return 0;
}
