#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 5;
const int M = 1e6 + 5;
const int mod = 1e9 + 7;
const int m1 = 1e9 + 7;
const int m2 = 1e9 + 9;
const int p1 = 402653189;
const int p2 = 1610612741;
const int LN = 22;
int powermodm(int x, int n, int M) {
  int result = 1;
  while (n > 0) {
    if (n % 2 == 1) result = (result * x) % M;
    x = (x * x) % M;
    n = n / 2;
  }
  return result;
}
int power(int _a, int _b) {
  int _r = 1;
  while (_b) {
    if (_b % 2 == 1) _r = (_r * _a);
    _b /= 2;
    _a = (_a * _a);
  }
  return _r;
}
int gcd(int a, int b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
int lcm(int a, int b) { return (max(a, b) / gcd(a, b)) * min(a, b); }
int dx[4] = {-1, 1, 0, 0};
int dy[4] = {0, 0, 1, -1};
int pw[LN], fact[N], invfact[N];
void pre() {
  fact[0] = 1;
  for (int i = 0; i < LN; i++) pw[i] = power(2, i);
  for (int i = 1; i < N; i++) fact[i] = (i * fact[i - 1]) % mod;
  for (int i = 0; i < N; i++) invfact[i] = powermodm(fact[i], mod - 2, mod);
}
int dp[(1 << 20) + 10];
int cnt[20][20], tot[20];
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  pre();
  int i, j;
  int n, m;
  cin >> n >> m;
  string s;
  cin >> s;
  for (i = 0; i < s.length() - 1; i++) {
    if (s[i] != s[i + 1]) {
      tot[s[i] - 'a']++;
      tot[s[i + 1] - 'a']++;
      cnt[s[i] - 'a'][s[i + 1] - 'a']++;
      cnt[s[i + 1] - 'a'][s[i] - 'a']++;
    }
  }
  for (i = 1; i < pw[m]; i++) {
    dp[i] = 1e9;
    vector<int> active;
    for (j = 0; j < m; j++) {
      if (pw[j] & i) active.push_back(j);
    }
    int sz = active.size();
    for (auto it : active) {
      int val = i - pw[it];
      int tans = 0;
      for (auto it1 : active) tans += cnt[it][it1];
      dp[i] = min(dp[i], dp[val] + (2 * tans - tot[it]) * sz);
    }
  }
  cout << dp[pw[m] - 1];
  return 0;
}
