#include <bits/stdc++.h>   using namespace std;   int main() {     priority_queue<int, vector<int>, greater<int> > a;     int temp;     int n, i;     int sum = 0;     scanf( %d , &n);     for (i = 1; i <= n; i++) {       scanf( %d , &temp);       a.push(temp);     }     temp = -1;     while (!a.empty()) {       if (a.top() == temp) {         printf( -1 n );         return 0;       } else if (a.top() == 0) {         a.pop();       } else {         temp = a.top();         a.pop();         if (!a.empty() && a.top() == temp) {           sum++;           a.pop();         }       }     }     printf( %d n , sum);     return 0;   }   