#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){
    ios::sync_with_stdio(false);
    int T, N=0, a=0, mark[1001];
    bool flag;
    std::fill(mark, mark+1000, 0);
    vector<int> twos;
    cin>>T;
    if (T < 1 || T > 100) return -1;
    for(register int i = 0; i < T; i++){
        flag = false;
        twos.clear();
        std::fill(mark, mark+1000, 0);
        cin>>N;
        if (N < 1 || N > 1000) continue;
        if (N < 4) {
            cout<<-1<<endl;
            continue;
        }
        for(register int j = 0; j < N; j++){
            cin>>a;
            if (a < 1 || a > 1000){
                flag = true;
                break;
            }
            mark[a]++;
            if (mark[a]==2){
                twos.push_back(a);
            }
        }
        if (flag) continue;
        if (twos.size() == 1||twos.size()==0){
            cout<<-1<<endl;
        }
        else{
            sort(twos.begin(), twos.end(), std::greater<int>());
            cout<<twos[0] * twos[1]<<endl;
        }
    }
    return 0;
}