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.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		Map<Long , Integer> map = new HashMap<>();
		int i , j , n;
		n = scan.nextInt();
		for (i = 0;i < n;i ++) {
			long value = scan.nextLong();
			if (!map.containsKey(value)) {
				map.put(value , 1);
			} else {
				map.put(value , map.get(value) + 1);
			}
		}
		long ans = 0;
		List<Long> list = new ArrayList<>();
		for (Map.Entry<Long , Integer> entry : map.entrySet()) {
			if (entry.getValue() >= 4) {
				long temp = entry.getKey();
				if (temp * temp > ans) {
					ans = temp * temp;
				}
			}
			if (entry.getValue() >= 2) {
				list.add(entry.getKey());
			}
		}
		Collections.sort(list);
		if (list.size() > 1) {
			long temp = list.get(list.size() - 1) * list.get(list.size() - 2);
			if (temp > ans) {
				ans = temp;
			}
		}
		System.out.println(ans);
		
	}
	
	
}