#include "bits/stdc++.h"
using namespace std;
#define int long long
#define REP(i,n) for(int i = 0;i < (int)(n);i++)
#define RREP(i,n) for(int i = (int)n-1;i >= 0;i--)
#define FOR(i,s,n) for(int i = s;i < (int)n;i++)
#define RFOR(i,s,n) for(int i = (int)n-1;i >= s;i--)
#define ALL(a) a.begin(),a.end()
template<class T>inline void out(T t){cout<<t<<"\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout<<t<<" ";out(ts...);}
template<class T>inline bool CHMAX(T&a,T b){if(a<b){a = b;return true;}return false;}
template<class T>inline bool CHMIN(T&a,T b){if(a>b){a = b;return true;}return false;}
constexpr long long INF = 1e18;

#define ull unsigned long long

ull gcd(ull x,ull y)
{
	if(x==0) return y;
	return gcd(y%x,x);
}
int lcm(int x, int y)
{
	return (x * y / gcd(x, y));
}

vector<pair<long long, long long> > prime_factorize(long long n) {
	vector<pair<long long, long long> > res;
	for (long long p = 2; p * p <= n; ++p) {
		if (n % p != 0) continue;
		int num = 0;
		while (n % p == 0) { ++num; n /= p; }
		res.push_back(make_pair(p, num));
	}
	if (n != 1) res.push_back(make_pair(n, 1));
	return res;
}

long long mod_pow(long long x, long long n) {
	long long res = 1;
	while (n > 0) {
		if (n & 1) res = res * x;
		x = x * x;
		n >>= 1;
	}
	return res;
}


signed main(){
	int N;
	cin >> N;
	vector<int>a(1ll << 20);
	vector<int>ok(1ll << 20);
	REP(i,N){
		cin >> a[i];
		if(a[i] == -1)ok[i] = 1;
	}
	REP(k,20){
		REP(s,N){
			if(!((s>>k)&1)){
				if(ok[s^(1ll<<k)])ok[s]=1;
				else if(a[s] == -1)continue;
				else a[s]+=a[s^(1ll<<k)];
			}
		}
	}
	int g = -1;
	REP(i,N){
		if(ok[i])continue;
		if(g == -1)g = a[i];
		else g = gcd(g,a[i]);
	}
	if(g == -1)cout << -1 << endl;
	else cout << g*g << endl;
}