#include <bits/stdc++.h>
#define Khashwaa() ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define ulld unsigned long long
#define itn int
#define endl '\n'
#define lld long long
#define fix(n) fixed << setprecision(n)
#define gcd(x,y) __gcd(x, y)
#define lcm(x,y) (x/(__gcd(x,y))*y)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define Uint unsigned int
#define tostr(n,s) ostringstream St;St<<n;string s=St.str();
void mini(lld &a, lld b) { a = std::min(a, b); }
void maxi(lld &a, lld b) { a = std::max(a, b); }
int dx[] = { 1, 0, -1, 0 };
int dy[] = { 0, -1, 0, 1 };
const lld N = 1e6 + 7;
const lld MOD = 1e9 + 7;
using namespace std;
void _Function()
{
int n;cin >> n;
vector<int> a(n);
for (int i = 0;i < n; ++i) cin >> a[i];
int x = -1;
for (int i = 0;i < n; ++i){
if(a[i] != i){
x = max(x , a[i] & i);
}
}
cout << x << endl;
}
int32_t main()
{
Khashwaa();
int t = 1;cin >> t;
while(t--){
_Function();
}
}
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