#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string s;
  cin >> s;
  vector<bool> unused(26, true);
  if (n > 26) {
    cout << "-1";
    return 0;
  }
  int sol = 0;
  for (int i = 0; i < n; i++) {
    if (unused.at(s.at(i) - 'a'))
      unused.at(s.at(i) - 'a') = false;
    else
      sol++;
  }
  cout << sol;
  return 0;
}
