#include <bits/stdc++.h>
using namespace std;
const int N = 20;
const int M = 1e5 + 55;
pair<int, int> dp[(1 << N) + 5];
int dis[N][N];
int My[(1 << N) + 5][N];
int taken[(1 << N) + 5][N];
char s[M];
int sum[N];
int lg[(1 << N) + 55];
int m, n;
pair<int, int> bt(int mask) {
  if (mask == (1 << n) - 1) return {0, 0};
  pair<int, int>& ret = dp[mask];
  if (ret.first != -1) return ret;
  ret = {10000000, 0};
  int cur = 0;
  pair<int, int> nxt;
  for (int v = 0; v < n; v++) {
    if ((mask & (1 << v)) == 0) {
      nxt = bt(mask + (1 << v));
      cur = nxt.second - My[mask][v];
      cur += nxt.first + taken[mask][v];
      if (cur < ret.first)
        ret = {cur, nxt.second + taken[mask][v] - My[mask][v]};
    }
  }
  return ret;
}
void prep(int mask, int i, int l, int pre) {
  if (i == n - 1) {
    taken[mask][l] = pre;
    My[mask][l] = 0;
    return;
  }
  if (My[mask][l] != -1) return;
  for (int j = 0; j < n; j++) {
    if ((mask & (1 << j)) == 0 && j != l) {
      prep(mask + (1 << j), i + 1, l, pre + dis[l][j]);
      My[mask][l] = My[mask + (1 << j)][l] + dis[l][j];
    }
  }
  taken[mask][l] = pre;
}
int main() {
  memset(dp, -1, sizeof dp);
  memset(My, -1, sizeof My);
  scanf("%d %d", &m, &n);
  scanf("%s", s);
  for (int i = 0; i < m - 1; i++) {
    if (s[i] != s[i + 1]) {
      dis[s[i] - 'a'][s[i + 1] - 'a']++;
      sum[s[i] - 'a']++;
      sum[s[i + 1] - 'a']++;
      dis[s[i + 1] - 'a'][s[i] - 'a']++;
    }
  }
  for (int i = 0; i < n; i++) lg[(1 << i)] = i;
  int ms;
  for (int l = 0; l < n; l++) {
    for (int mask = (1 << n) - 1; mask >= 0; mask--) {
      if (mask & (1 << l)) continue;
      mask += (1 << l);
      ms = mask | (mask + 1);
      mask -= (1 << l);
      ms -= (1 << l);
      if (ms >= (1 << n)) {
        taken[mask][l] = sum[l];
        My[mask][l] = 0;
        continue;
      }
      My[mask][l] = My[ms][l] + dis[lg[mask ^ ms]][l];
      taken[mask][l] = taken[ms][l] - dis[l][lg[mask ^ ms]];
    }
  }
  cout << bt(0).first << endl;
  return 0;
}
