#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  int flag = 0;
  for (int i = 1; i < n; i++) {
    if ((a[i] == a[i - 1]) && (a[i] != 0)) flag++;
    if ((i < n - 1) && (a[i] == a[i - 1]) && (a[i] == a[i + 1]) &&
        (a[i] != 0)) {
      flag = -1;
      break;
    }
  }
  cout << flag;
  return 0;
}
