#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using pii = pair<int, int>;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define repr(i, n) for (int i = (int)(n - 1); i >= 0; --i)

ll gcd(ll x, ll y) {
    if (x > y) swap(x, y);
    return x == 0 ? y : gcd(y % x, x);
}
int main() {
    int n;
    cin >> n;
    vector<ll> a(n);
    rep(i, n) cin >> a[i];
    for (int i = 1; i < n; i <<= 1) {
        for (int j = 0; j < n; j++) {
            if (i & j) {
                if (a[i ^ j] < 0 || a[j] < 0) {
                    a[i ^ j] = -1;
                } else {
                    a[i ^ j] += a[j];
                }
            }
        }
    }
    ll r = 0;
    rep(i, n) {
        if (a[i] >= 0) {
            r = gcd(r, a[i] * a[i]);
        }
    }
    cout << (r == 0 ? -1 : r) << endl;
    return 0;
}