#include <iostream>
#include <algorithm>
#include <vector>

using lint = long long;

template <class T>
T gcd(T n, T m) {
    while (m != 0) {
        n %= m;
        std::swap(n, m);
    }
    return n;
}

void solve() {
    int n;
    std::cin >> n;

    std::vector<lint> xs(n);
    for (auto& x : xs) std::cin >> x;

    int i;
    for (i = n - 1; i >= 0; --i) {
        if (xs[i] != -1) break;
    }

    if (i < 0) {
        std::cout << -1 << std::endl;
        return;
    }

    for (auto& x : xs) {
        if (x == -1) x = xs[i];
    }

    for (int k = 0; k < 20; ++k) {
        for (int b = n - 1; b >= 0; --b) {
            if ((b >> k) & 1) xs[b ^ (1 << k)] += xs[b];
        }
    }

    lint g = 0;
    for (auto x : xs) g = gcd(g, x);
    std::cout << g * g << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}