#include <bits/stdc++.h>
using std::cin;
using std::cout;
using std::set;
using std::string;
int main() {
  set<char> X;
  int n;
  cin >> n;
  string s;
  cin >> s;
  for (int i = 0; i < s.size(); i++) X.insert(s[i]);
  if (s.size() < 27)
    cout << n - X.size();
  else
    cout << -1;
  return 0;
}
