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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)

import java.util.*;

public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		Integer[] a = new Integer[n];
		
		for(int i=0; i<a.length; i++) {
			a[i] = sc.nextInt();
		}
		sc.close();
		
		Arrays.sort(a, Collections.reverseOrder());
		
		boolean br = false;
		int height = 0;
		int width = 0;
		
		for(int i=0; i<a.length; i++) {
			int stick = a[i];
			int count = 0;
			
			while(i+1<a.length && a[i+1]==stick ) {
				count++;
				i++;
			}
			
			if(count>=3) {
				height = stick;
				width = stick;
				
				br = true;
			}else if(count==1 || count==2) {
				if(height != 0) {
					width = stick;
					br = true;
				}else {
					height = stick;
				}
			}
			
			
			if(br) {
				break;
			}
		}
		
		System.out.println(height * width);
		
	}

}