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.io.File;
import java.io.IOException;
import java.util.*;
import java.util.Map.Entry;

public class Main{
	
	public static void main(String[] args) throws IOException{
		//File file = new File("input.txt");
		//Scanner in = new Scanner(file);
		
		Scanner in = new Scanner(System.in);
		
		int N = in.nextInt();
		int[] a = new int[N];
		
		for(int i = 0; i < N; i++){
			a[i] = in.nextInt();
		}
		
		Arrays.sort(a);
		
		int max1 = 0, max2 = 0, index = -1;
		for(int i = N - 1; i >= 1; i--){
			if(a[i] == a[i-1] && a[i] > max1){
				max1 = a[i];
				index = i - 1;
			}else if(a[i] == a[i-1] && a[i] <= max1 && a[i] > max2 && i != index){
				max2 = a[i];
			}
		}
		
		System.out.println(max1 * max2);
	}
}