#include <bits/stdc++.h>   using namespace std;   int main() {     int t;     cin >> t;     while (t--) {       int i, n, j;       cin >> n;       int arr[n], a[n];       set<int> s1, s2;       int flag = 0, p = 0;       for (i = 0; i < n; i++) {         cin >> arr[i];         a[i] = arr[i];         s2.insert(arr[i]);         if (arr[i] == 0) flag = 1;       }       sort(a, a + n);       if (flag == 1) {         p == 0;         for (i = 0; i < n; i++) {           s1.clear();           if (a[i] != 0) {             for (j = 0; j < n; j++) {               int p = (a[i] ^ arr[j]);               s1.insert(p);             }           }           if (s2 == s1) {             p = 1;             cout << a[i] <<   n ;             break;           }         }         if (p == 0) cout <<  -1 n ;       } else {         p = 0;         for (i = 1; i < 1025; i++) {           s1.clear();           for (j = 0; j < n; j++) {             s1.insert(i ^ arr[j]);           }           if (s2 == s1) {             p = 1;             cout << i <<   n ;             break;           }         }         if (p == 0) cout <<  -1 n ;       }     }   }   