import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;


public class CodeChef_Java {

    public static void main(String[] args) {
        ArrayList<Integer> twos = new ArrayList<>();
        int mark[] = new int[1001];
        int T = 0, N=0, a=0;
        Scanner sc = new Scanner(System.in);
        if (sc.hasNextInt())
            T = sc.nextInt();

        if (T < 1 || T > 1000) return;

        for(int i = 0; i < T; i++){
            twos.clear();
            for(int j = 0; j < 1001; j++){
                mark[i] = 0;
            }
            if (sc.hasNextInt())
                N = sc.nextInt();

            if (N < 1 || N > 1000) return;

            for(int j=0; j < N; j++){
                if (sc.hasNextInt())
                    a = sc.nextInt();
                if (a < 1 || a > 1000){
                    return;
                }
                mark[a]++;
                if (mark[a] == 2){
                    twos.add(a);
                }
            }

            if (twos.size() == 1||twos.isEmpty()){
                System.out.println(-1);
            }
            else{
                Collections.sort(twos, Collections.reverseOrder());
                System.out.println(twos.get(0)*twos.get(1));
            }
        }
    }

}