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.*;
public class Main{

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		
		int N = sc.nextInt();
		
		PriorityQueue<Long> queue = new PriorityQueue<Long>(Collections.reverseOrder());
		HashSet<Long> has = new HashSet<Long>();
		
		queue.add((long) 0); queue.add((long) 0);
		for(int i = 1; i<=N; i++) {
			long n = sc.nextInt();
			if(has.contains(n)) {
				queue.add((n));
				has.remove(n);
			}
			else {
				has.add(n);
			}
		}
		
		System.out.println(queue.poll()*queue.poll());
		
	}

}