#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string& second) { return '"' + second + '"'; }
string to_string(const char* second) { return to_string((string)second); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
const int N = 2e5 + 5;
vector<vector<int>> cnt(20, vector<int>(20));
vector<int> dp(1 << 20, 1000000000);
vector<vector<int>> d(1 << 20, vector<int>(20));
void run_case() {
  int n, m;
  cin >> n >> m;
  string second;
  cin >> second;
  for (int i = 0; i + 1 < n; i++) {
    cnt[second[i] - 'a'][second[i + 1] - 'a']++;
    cnt[second[i + 1] - 'a'][second[i] - 'a']++;
  }
  for (int i = 0; i < m; i++) {
    for (int j = 0; j < m; j++) {
      d[(1 << j)][i] = cnt[i][j];
    }
  }
  for (int msk = 1; msk < (1 << m); msk++) {
    for (int i = 0; i < m; i++) {
      d[msk][i] = d[msk & (msk - 1)][i] + d[msk & -msk][i];
      42;
    }
  }
  dp[0] = 0;
  for (int msk = 1; msk < (1 << m); msk++) {
    for (int i = 0; i < m; i++) {
      if (((msk >> i) & 1) == 1) {
        int pos = __builtin_popcount(msk);
        dp[msk] = min(dp[msk],
                      dp[msk ^ (1 << i)] + pos * (d[msk ^ (1 << i)][i] -
                                                  d[((1 << m) - 1) ^ msk][i]));
      }
    }
  }
  cout << dp[(1 << m) - 1] << endl;
}
auto clk = clock();
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t;
  t = 1;
  while (t--) {
    run_case();
  }
  return 0;
}
