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.Collections;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();

		ArrayList<Integer> rods = new ArrayList<>();
		for(int i = 0; i < N; i++) {
			rods.add(sc.nextInt());
		}
		Collections.sort(rods);

		int a = 0;
		int b = 0;
		boolean flag = true;
		for(int i = rods.size() - 1; i >= 0; i--) {
			if(a == rods.get(i)) {
				rods.remove(i+1);
				rods.remove(i);
				flag = false;
				break;
			} else {
				a = rods.get(i);
			}
		}

		if(flag) {
			a = 0;
		}
		flag = true;

		for(int i = rods.size() - 1; i >= 0; i--) {
			if(b == rods.get(i)) {
				rods.remove(i+1);
				rods.remove(i);
				flag = false;
				break;
			} else {
				b = rods.get(i);
			}
		}
		if(flag) {
			b = 0;
		}

		System.out.println(a * b);

	}
}