#include <bits/stdc++.h>
using namespace std;
const int K = 20, N = 1 << 20;
int dp[N];
int pop[K][N];
int cnt[K][K];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  for (int i = 1; i < n; i++) {
    int aa = s[i] - 'a';
    int bb = s[i - 1] - 'a';
    cnt[aa][bb]++;
    cnt[bb][aa]++;
  }
  for (int i = 0; i < k; i++) {
    for (int mask = 1; mask < (1 << k); mask++) {
      int first = __builtin_ctz(mask);
      pop[i][mask] = pop[i][mask ^ (1 << first)] + cnt[i][first];
    }
  }
  for (int mask = 1; mask < (1 << k); mask++) {
    dp[mask] = 1e9 + 7;
    for (int b = 0; b < k; b++) {
      if ((mask & (1 << b)) == 0) continue;
      int left = pop[b][mask ^ (1 << b)];
      int right = pop[b][((1 << k) - 1) ^ mask];
      dp[mask] = min(dp[mask], dp[mask ^ (1 << b)] +
                                   (left - right) * __builtin_popcount(mask));
    }
  }
  cout << dp[(1 << k) - 1] << endl;
}
