Untitled

#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    //cout << (6&4) << endl;
    int t,n;
    cin >> t;
    for(int i=0;i<t;++i){
        cin >> n ;
        int arr[n];
        for(int j=0;j<n;++j){
            cin >> arr[j];
        }
        int mx = -1,x;
        for(int j=0;j<n;++j){
            if(arr[j] != j){
                x = arr[j] & j;
                if(x > mx)
                    mx = x;
            }
        }
        cout << mx << endl;
    }
    return 0;
}