#include <iostream>
#include <cstring>
#include <cmath>
#include <vector>
#include <algorithm>
using namespace std;
int n, m, dp[1 << 20], d[20][20];
string s;

int f(int b) {
  if(b == (1 << m) - 1)
    return 0;
  if(dp[b] != -1)
    return dp[b];
  int mn = 99999999;
  for(int i = 0; i < m; ++i)
    if(~b & (1 << i)) {
      int c = 0, cnt = 1;
      vector<int> v;
      for(int j = 0; j < 20; ++j)
        if(b & (1 << j))
          v.push_back(d[i][j]);
      sort(v.begin(), v.end());
      for(int j = 0; j < v.size(); ++j)
        c += v[v.size() - j - 1] * (j + 1);
      mn = min(mn, c + f(b | (1 << i)));
    }
  return dp[b] = mn;
}
int main() {
  cin >> n >> m >> s;
  memset(dp, -1, sizeof dp);
  n = unique(s.begin(), s.end()) - s.begin();
  for(int i = 1; i < n; ++i)
    ++d[s[i] - 'a'][s[i - 1] - 'a'], ++d[s[i - 1] - 'a'][s[i] - 'a'];
  cout << f(0) << endl;
}