#include <bits/stdc++.h>
using namespace std;
long long mat[21][21];
long long n, m;
long long power(long long a, long long b, long long m = 1000000007) {
  if (b == 0) return 1;
  if (b == 1) return a;
  long long res = power(a, b / 2, m);
  res = (res * res) % m;
  if (b & 1) res = (res * a) % m;
  return res;
}
long long modinv(long long a, long long m = 1000000007) {
  return power(a, m - 2, m);
}
long long add(long long a, long long b, long long m = 1000000007) {
  return (a + b) % m;
}
long long sub(long long a, long long b, long long m = 1000000007) {
  return (a - b + m) % m;
}
long long mul(long long a, long long b, long long m = 1000000007) {
  return (a * b) % m;
}
void solve() {
  cin >> n >> m;
  string s;
  cin >> s;
  memset(mat, 0, sizeof(mat));
  long long i, a, b, dp[1 << m], x, j, k;
  for (i = 0; i < (1 << m); i++) dp[i] = 1e9;
  dp[0] = 0;
  for (i = 1; i < n; i++) {
    if (s[i] == s[i - 1]) continue;
    a = s[i] - 'a';
    b = s[i - 1] - 'a';
    mat[a][b]++;
    mat[b][a]++;
  }
  for (i = 0; i < (1 << m); i++) {
    for (j = 0; j < m; j++) {
      long long val = 1 << j;
      long long cnt = __builtin_popcount(i), ans = 0;
      if ((val & i) == 0) continue;
      for (k = 0; k < m; k++) {
        x = (1 << k);
        if ((x & i) == 0)
          ans -= mat[j][k];
        else
          ans += mat[k][j];
      }
      ans *= cnt;
      dp[i] = min(dp[i], dp[i ^ val] + ans);
    }
  }
  cout << dp[(1 << m) - 1] << "\n";
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t = 1;
  while (t--) solve();
  return 0;
}
