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){
		Scanner scan = new Scanner(System.in);
		
		// input
		int num = scan.nextInt();
		long[] edges = new long[100000];
		for(int i = 0; i < num; i++){
			edges[i] = scan.nextLong();
		}
		
		// search
		Arrays.sort(edges);
		long memory = edges[99999];
		int count = 1;
		long firstLen = 0;
		long secondLen = -1;
		for(int i = 99998; i >= 0; i--){
			if(memory == edges[i]){
				count++;
				if(count == 2){
					if(edges[i] > firstLen){
						secondLen = firstLen;
						firstLen = edges[i];
						count = 0;
					} else if(edges[i] > secondLen){
						secondLen = edges[i];
						count = 0;
					}
				}
			} else {
				memory = edges[i];
				count = 1;
			}
		}
		
		// output
		long area = 0;
		if(firstLen * secondLen > 0){
			area = firstLen * secondLen;
		}
		System.out.println(area);
		
	}
	
}