#include <bits/stdc++.h>
using namespace std;
const long long inf = (long long)1e18;
const long long mod = (long long)1e9 + 7;
const double eps = (double)1e-9;
const double pi = acos(-1.0);
const int dx[] = {0, 0, 1, 0, -1};
const int dy[] = {0, 1, 0, -1, 0};
const int N = 20;
string s;
int n, m, cnt[N][N], f[1 << N], dp[1 << N];
int main() {
  cin.tie(NULL);
  cout.tie(NULL);
  ios_base::sync_with_stdio(false);
  cin >> n >> m >> s;
  for (int i = 0; i + 1 < (int)s.size(); i++) {
    int x = s[i] - 'a';
    int y = s[i + 1] - 'a';
    cnt[x][y]++;
    cnt[y][x]++;
  }
  for (int mask = 0; mask < (1 << m); mask++) {
    for (int i = 0; i < m; i++) {
      if (!(mask >> i & 1)) continue;
      for (int j = 0; j < m; j++) {
        if (mask >> j & 1) continue;
        f[mask] += cnt[i][j];
      }
    }
  }
  for (int mask = 1; mask < (1 << m); mask++) {
    dp[mask] = (int)1e9;
    for (int i = 0; i < m; i++) {
      if (!(mask >> i & 1)) continue;
      dp[mask] = min(dp[mask], dp[mask ^ (1 << i)] + f[mask]);
    }
  }
  cout << dp[(1 << m) - 1];
  return 0;
}
