#include <bits/stdc++.h>
using namespace std;
const int M = 21, N = 1e5 + 5, INF = 123456789;
int dp[(1 << M)], sum[(1 << M)][M];
int cnt[25][25], n, m;
char s[N];
int main() {
  scanf("%d%d", &n, &m);
  scanf("%s", (s + 1));
  int len = strlen(s + 1);
  for (int i = 2; i <= len; i++) {
    cnt[s[i - 1] - 'a' + 1][s[i] - 'a' + 1]++;
    cnt[s[i] - 'a' + 1][s[i - 1] - 'a' + 1]++;
  }
  for (int i = 1; i <= m; i++) cnt[i][i] = 0;
  memset(dp, 0x7f, sizeof(dp));
  dp[0] = 0;
  for (int S = 0; S < (1 << m); S++) {
    int summ = 0, T = (S ^ ((1 << m) - 1));
    bool ex[M];
    for (int x = 1; x <= m; x++) {
      if (S & (1 << (x - 1))) {
        ex[x] = 1;
        summ++;
      } else
        ex[x] = 0;
    }
    for (int x = 1; x <= m; x++) {
      if (ex[x]) continue;
      int tmpsum = 0;
      for (int y = 1; y <= m; y++) {
        if (!ex[y]) {
          tmpsum -= cnt[x][y];
        }
        if (ex[y]) {
          tmpsum += cnt[x][y];
        }
      }
      dp[S | (1 << (x - 1))] = min(1ll * dp[S | (1 << (x - 1))],
                                   dp[S] + 1ll * (summ + 1) * (tmpsum));
    }
  }
  printf("%d", dp[(1 << m) - 1]);
}
