#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
// using namespace atcoder;
// using mint = modint1000000007;
// const int mod = 1000000007;
// using mint = modint998244353;
// const int mod = 998244353;
// const int INF = 1e9;
// const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i, l, r) for (int i = (l); i < (r); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rrep2(i, l, r) for (int i = (r)-1; i >= (l); --i)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define P pair<int, int>
template<typename A, typename B> inline bool chmax(A &a, const B &b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A &a, const B &b) { if (a > b) { a = b; return true; } return false; }
template<typename T>
std::vector<T> zeta(int n, std::vector<T> z) {
	for (int j = 0; j < n; ++j) {
		for (int i = 0; i < (1 << n); ++i) {
			if (1 & (i >> j)) {
				z[i - (1 << j)] += z[i];
			}
		}
	}
	return z;
}
template<typename T>
std::vector<T> mobius(int n, std::vector<T>z) {
	for (int j = n - 1; j >= 0; --j) {
		for (int i = (1 << n) - 1; i >= 0; --i) {
			if (1 & (i >> j)) {
				z[i - (1 << j)] -= z[i];
			}
		}
	}
	return z;
}

#ifndef KWM_T_BASE_INTEGR_HPP
#define KWM_T_BASE_INTEGR_HPP

#include <vector>

/**
 * @brief ローカル環境用(MSC)int128
 */
#ifdef _MSC_VER
using int128 = long long;
#else
using int128 = __int128;
#endif

#endif // KWM_T_BASE_INTEGR_HPP


template<class T>
constexpr T GCD(T a, T b) {
	if (a < 0) a = -a;
	if (b < 0) b = -b;
	while (b) {
		T t = a % b;
		a = b;
		b = t;
	}
	return a;
}
int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int n; cin >> n;
	vector<long long>a(n);
	rep(i, n)cin >> a[i];
	rep(i, n)if (a[i] == -1)a[i] = 0;
	a.resize(1 << 20);
	a = zeta(20, a);
	rep(i, a.size()) a[i] *= a[i];
	a = mobius(20, a);
	int128 ans = 0;
	rep(i, a.size())ans = GCD(ans, (int128)a[i]);
	if (ans == 0) ans = -1;
	cout << (long long)ans << endl;
	return 0;
}