#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, alph[26] = {0}, counter = 0;
  string s;
  cin >> n >> s;
  if (n > 26) {
    cout << -1;
    return 0;
  }
  for (int i = 0; i < n; i++) alph[s[i] - 97]++;
  for (int i = 0; i < 26; i++)
    if (alph[i]) counter += alph[i] - 1;
  cout << counter;
  return 0;
}
