#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  string inputString;
  int len;
  cin >> len >> inputString;
  if (len > 26) {
    cout << -1;
    return 0;
  }
  bool alphabets[26];
  memset(alphabets, false, 26);
  int count = 0;
  for (int i = 0; i < len; i++) {
    if (alphabets[inputString[i] - 97] == true) {
      count++;
    } else {
      alphabets[inputString[i] - 97] = true;
    }
  }
  cout << count;
  return 0;
}
