#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, mncnt = 1000000000, cnt;
  string a;
  string c = "ACTG";
  cin >> n >> a;
  for (int i = 0; i <= n - 4; i++) {
    cnt = 0;
    string b = a.substr(i, 4);
    for (int j = 0; j < 4; j++) {
      int x = c[j] - 'A';
      int y = b[j] - 'A';
      if (abs(y - x) <= 13)
        cnt += abs(y - x);
      else
        cnt += 26 - (abs(y - x));
    }
    mncnt = min(mncnt, cnt);
  }
  cout << mncnt << endl;
}
