Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

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

import org.xml.sax.HandlerBase;

public class Main {

	public static void main(String[] args) {
		new Main().execute();
	}

	public void execute() {
		Scanner sc = new Scanner(System.in);
		final int N = sc.nextInt();
		ArrayList<Integer> sides = new ArrayList<>();
		HashSet<Integer> bar = new HashSet<>();

		for (int i = 0; i < N; i++) {
			int ai = sc.nextInt();
			if (bar.contains(ai)) {
				sides.add(ai);
				bar.remove(ai);
			} else {
				bar.add(ai);
			}
		}

		Collections.sort(sides);

		int sz = sides.size();
		if (sz < 2) {
			System.out.println(0);
		} else {
			System.out.println((long) sides.get(sz - 1) * (long) sides.get(sz - 2));
		}

		sc.close();
	}
}