#include <bits/stdc++.h>   using namespace std;   const int MAXN = 1e6 + 22;   const int INF = 1e9 + 2;   long long n, ans, mark[MAXN];   char str[MAXN];   int main() {     cin >> n;     cin >> str;     for (int i = 0; i < n; i++) {       if (mark[str[i]] == 0) {         mark[str[i]] = 1;         ans++;       }     }     if (n > 26)       cout << -1;     else       cout << n - ans;     return 0;   }   