#include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <algorithm>
#include <utility>
#include <map>
#include <set>
#include <string>
#include <cstring>
#include <queue>
#define rf freopen("in.in", "r", stdin)
#define wf freopen("out.out", "w", stdout)
#define rep(i, s, n) for(int i=int(s); i<=int(n); ++i)
using namespace std;
const int mx = 1e5 + 10, mod = 1e9+7;

map<int, int> numOfSticksLen;

int main() {

	//rf;// wf;
	ios::sync_with_stdio(0);

	int t;
	cin >> t;
	while(t--) {

		int n;
		numOfSticksLen.clear();

		cin >> n;
		rep(i, 1, n) {

			int a;
			cin >> a;
			numOfSticksLen[a]++;
		}

		map<int, int>::reverse_iterator rit = numOfSticksLen.rbegin();

		int best=0, secondBest=0;

		while(rit != numOfSticksLen.rend() and (!best or !secondBest)) {

			if(rit->second >= 4) {
				if(!best)
					best = secondBest = rit->first;
				else
					secondBest = rit->first;
				break;
			}
			else if(rit->second >= 2 and !best) {
				best = rit->first;
			}
			else if(rit->second >=2 and !secondBest) {
				secondBest = rit->first;
				break;
			}

			rit++;
		}

		if(best and secondBest)
			cout << best*secondBest << endl;
		else
			cout << -1 << endl;
	}
	return 0;
}