#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 123;
const int M = 1 << 20;
const int oo = 1e9 + 7;
int n, m, cnt[20][20], f[M];
string s;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie();
  cout.tie();
  cin >> n >> m;
  cin >> s;
  for (int i = 1; i < n; ++i) {
    int x = s[i - 1] - 'a', y = s[i] - 'a';
    ++cnt[x][y];
    ++cnt[y][x];
  }
  int k = 1 << m;
  for (int i = 1; i < k; ++i) {
    f[i] = oo;
    int t = __builtin_popcount(i);
    for (int j = 0; j < m; ++j)
      if (i & (1 << j)) {
        int x = i ^ (1 << j), s = f[x];
        for (int b = 0; b < m; ++b)
          if (!(i & (1 << b)))
            s += -t * cnt[j][b];
          else if (b != j)
            s += t * cnt[j][b];
        f[i] = min(f[i], s);
      }
  }
  cout << f[k - 1];
  return 0;
}
