#include <bits/stdc++.h>
using namespace std;
void Solve() {
  int n;
  cin >> n;
  string str;
  cin >> str;
  if (n > 26) {
    cout << -1;
    return;
  }
  int freq[26] = {0};
  for (long long int i = 0; i <= n - 1; i++) freq[str[i] - 'a']++;
  int ans = 0;
  for (long long int i = 0; i <= 25; i++)
    if (freq[i]) ans += (freq[i] - 1);
  cout << ans;
}
int main(int argc, char const *argv[]) {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(0);
  ;
  int T = 1;
  while (T--) Solve();
  return 0;
}
