#include <bits/stdc++.h>
using namespace std;
const int N = 2e6 + 100;
const int M = 30;
const long long mod = 1e9 + 7;
const long long MOD = 998244353;
const int P = 1336;
const long double eps = 0.000000001;
const int inf = 1e9 + 7;
mt19937 gen(chrono::high_resolution_clock::now().time_since_epoch().count());
int dp[N];
int a[M][M];
int b[M][N];
int add[N], pos[N];
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  srand(time(0));
  int n, m;
  cin >> n >> m;
  string s;
  cin >> s;
  for (int i = 0; i < n; i++) {
    int c = int32_t(s[i] - 'a');
    if (i + 1 < n) {
      int x = int32_t(s[i + 1] - 'a');
      a[c][x]++;
    }
    if (i) {
      int x = int32_t(s[i - 1] - 'a');
      a[c][x]++;
    }
  }
  for (int i = 0; i < (1 << m); i++) {
    dp[i] = inf;
  }
  dp[0] = 0;
  for (int mask = 0; mask < (1 << m); mask++) {
    for (int i = 0; i < m; i++) {
      if (mask & (1 << i)) {
        pos[mask] = i;
        break;
      }
    }
  }
  for (int i = 0; i < m; i++) {
    for (int mask = 1; mask < (1 << m); mask++) {
      int p = pos[mask];
      b[i][mask] = b[i][mask ^ (1 << p)] + a[i][p];
    }
  }
  for (int mask = 0; mask < (1 << m); mask++) {
    int t = (1 << m) - 1;
    int nm = (t ^ mask);
    for (int i = 0; i < m; i++) {
      if (mask & (1 << i)) continue;
      add[mask] += b[i][mask];
    }
  }
  for (int mask = 0; mask < (1 << m); mask++) {
    for (int i = 0; i < m; i++) {
      if (mask & (1 << i)) continue;
      dp[mask | (1 << i)] = min(dp[mask | (1 << i)], dp[mask] + add[mask]);
    }
  }
  cout << dp[(1 << m) - 1];
}
