#include <bits/stdc++.h>
#define ll long long
#define ishowspeed ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0)
#define pb push_back

using namespace std;

int main() {
    ishowspeed;
    int tt;
    cin>>tt;
    while(tt--){
        int n;
        cin>>n;
        int a[n];
        map <int,int> mp;
        for(int i=0;i<n;i++){
            cin>> a[i];
            mp[a[i]]++;
        }
        int mn=INT_MAX;
        for(int i=0;i<n;i++){
            if(mp[a[i]]%2==1){
                mn= min(mn,a[i]);
            }
        }
        if(mn==INT_MAX)
        cout<<-1<<endl;
        else
        cout<<mn<<endl;
    }
    return 0;       
}
