#include <bits/stdc++.h>   const long long int N = 2e5 + 10;   using namespace std;   int main() {     ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);     long long int n;     cin >> n;     string second;     cin >> second;     set<char> x;     if (n > 26) {       cout << -1 << endl;       return 0;     }     for (int i = 0; i < n; i++) {       x.insert(second[i]);     }     cout << n - x.size() << endl;   }   