#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int C = 20;
void add(int &x, int y) {
  x += y;
  while (x >= MOD) x -= MOD;
  while (x < 0) x += MOD;
}
int fix(int x) {
  while (x >= MOD) x -= MOD;
  while (x < 0) x += MOD;
  return x;
}
int pw(int a, int b) {
  int ret = 1;
  while (b) {
    if (b & 1) ret = 1ll * ret * a % MOD;
    b >>= 1;
    a = 1ll * a * a % MOD;
  }
  return ret;
}
const int MAXN = 1e5 + 10;
int n, cnt[C][C], dp[1 << C], cost[1 << C], m;
string s;
void solve() {
  cin >> n >> m;
  cin >> s;
  for (int i = 0; i + 1 < n; i++) cnt[s[i] - 'a'][s[i + 1] - 'a']++;
  for (int mask = 1; mask < 1 << m; mask++) {
    int v = __builtin_ctz(mask & -mask);
    cost[mask] = cost[mask ^ 1 << v];
    for (int u = 0; u < m; u++)
      if (u ^ v) {
        if (mask >> u & 1)
          cost[mask] -= cnt[v][u] + cnt[u][v];
        else
          cost[mask] += cnt[v][u] + cnt[u][v];
      }
  }
  for (int mask = 1; mask < 1 << m; mask++) {
    int mn = 1e9;
    for (int v = 0; v < m; v++)
      if (mask >> v & 1) mn = min(mn, dp[mask ^ 1 << v]);
    dp[mask] = mn + cost[mask];
  }
  cout << dp[(1 << m) - 1] << "\n";
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int te = 1;
  for (int w = 1; w <= te; w++) {
    solve();
  }
  return 0;
}
