B.cpp

/**************************************
                                      *
 * Author: Hadiur Rahman Nabil        *
                                      *
**************************************/
#include <bits/stdc++.h>
using namespace std;

typedef long int      li;
typedef long long     ll;
typedef long double   ld;
typedef vector <int>  vi;
typedef vector<ll>    vl;
typedef pair<int,int> pi;
typedef pair<ll,ll>   pll;

#define FAST          ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define pb            push_back
#define mp            make_pair
#define gcd(a,b)      __gcd(a,b)
#define FOR(i,n)      for(int i=0;i<n;i++)
#define rep(i,k,n)    for(int i=k; i<n; i++)
#define repp(i,k,n)   for(int i=k; i<=n; i++)6
#define endl          "\n"


const int             mod=1e5 + 10;
const double          PI=3.1415926535897932384626;
const int             mod_2 = 998244353;
const int             MAX=100005;
const double          PII=2.0*acos(0.0);


/**********************************************************************************************************************************************************************/

void solve()
{
  ll n;
  ll ans= -1;
  cin>>n;
  vector<ll>v(n);
  for(int i=0;i<n;i++)
    cin>>v[i];
    sort(v.begin(),v.end());
    for(int i=0;i<n;i++)
    {
    if(v[i]==v[i+1] and v[i]==v[i+2])
    {
      ans= v[i];
      break;
    }
  }
  cout<<ans<<"\n";
}



int main()
{
    FAST;

    #ifndef ONLINE_JUDGE
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
    #endif


    int t;
    cin>>t;
    while(t--)
    {
      solve();
     
    } 
   
  return 0;
}