#include <bits/stdc++.h>
using namespace std;
char st[100005];
int vis[100005];
int main() {
  long l, cnt = 0;
  scanf("%ld", &l);
  scanf("%s", st);
  if (l > 26) {
    cout << -1;
    return 0;
  }
  for (long i = 0; i < l + 10; i++) vis[i] = 0;
  for (long i = 0; i < l; i++) {
    if (vis[i] == 0) {
      vis[i] = 1;
      for (long j = i + 1; j < l; j++) {
        if (st[i] == st[j] && vis[j] == 0) {
          cnt++;
          vis[j] = 1;
        }
      }
    }
  }
  cout << cnt;
  return 0;
}
