#include <bits/stdc++.h>   using namespace std;   int n, ans, notOK;   map<int, int> M;   int main() {     cin >> n;     for (int i = 0, x; i < n; i++) cin >> x, M[x]++;     for (map<int, int>::iterator it = M.begin(); it != M.end(); it++) {       if (it->first) {         if (it->second == 2)           ans++;         else if (it->second > 2)           notOK = 1;       }     }     cout << (notOK ? -1 : ans);   }   