#include <vector>
#include <iostream>
using namespace std;
const int inf = 1012345678;
int gcd(int x, int y) {
	if (y == 0) return x;
	return gcd(y, x % y);
}
int main() {
	int N;
	cin >> N;
	int g = inf;
	for (int i = 0; i < N; ++i) {
		int x;
		cin >> x;
		if (x != -1) {
			if (g == inf) g = x;
			else g = gcd(g, x);
		}
	}
	if (g == inf) cout << -1 << endl;
	else cout << 1LL * g * g << endl;
	return 0;
}