#include <bits/stdc++.h>
using namespace std;
 
int main() {
    int t;
    cin>>t;
    int m = 1000000007;
    while(t--) {
        int n;
        cin>>n;
        int num, res;
        cin>>num;
        res = num;
        for(int i = 1; i < n; i++) {
            cin>>num;
            res = res&num;
        }
        if(__gcd(res, m) == 1)
            cout<<res<<"\n";
        else
            cout<<"-1\n";
    }
    return 0;
}