#include <stdio.h>
#include <algorithm>
#include <assert.h>
#include <set>
#include <map>
#include <complex>
#include <iostream>
#include <time.h>
#include <stack>
#include <stdlib.h>
#include <memory.h>
#include <bitset>
#include <math.h>
#include <string>
#include <string.h>
#include <queue>
#include <vector>

using namespace std;

const int MaxN = 5e3 + 10;
const int MOD = 1e9 + 7;
const int INF = 1e9;

int main() {
//  freopen("input.txt", "r", stdin);
  int t;
  cin >> t;
  while (t --> 0) {
    int n;
    cin >> n;
    vector < int > a(n);
    for (int i = 0; i < n; ++i) {
      cin >> a[i];
    }
    sort(a.rbegin(), a.rend());
    vector < pair < int, int > > v;
    for (int i = 0; i + 1 < n; ++i) {
      if (a[i] == a[i + 1]) {
        v.push_back(make_pair(a[i], a[i + 1]));
        ++i;
      }
    }
    if (v.size() < 2) {
      cout << "-1\n";
      continue;
    }
    cout << 1LL * v[0].first * v[1].first << '\n';
  }
  return 0;
}