#include<bits/stdc++.h>
using namespace std;
int a[200005], t[100005];
int main() {
	int n, ans = 0;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
		t[a[i]]++;
	}
	for (int i = 1; i <= 100000; i++) {
		if (t[i] == 1) {
			cout << -1;
			return 0;
		} else {
			ans += (t[i] + 2) / 3;
		}
	}
	cout << ans;
	return 0;
}