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: Wrong Answer)

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeSet;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int n = scanner.nextInt();
		List<Integer> list = new ArrayList<>();
		Set<Integer> tars = new TreeSet<>();

		for (int i = 0; i < n; i++) {
			int tmp = scanner.nextInt();
			if (list.contains(tmp)) {
				tars.add(tmp);
			}
			list.add(tmp);
		}

		long ans = 0;
		int count = 0;
		if (tars.size() >= 2) {
			ans = 1;
		} else if (tars.size() == 1) {
			for (int i : tars) {
				int cnt = 0;
				for (int j = 0; j < list.size(); j++) {
					if (list.get(i) == i) {
						cnt++;
					}
				}
				if (cnt >= 4) {
					System.out.println(i * i);
					return;
				}
			}

		}
		for (int i : tars) {
			if (count >= tars.size() - 2) {
				ans *= i;
			}
			count++;
		}

		System.out.println(ans);
	}
}