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.util.Map.Entry;
import java.util.Scanner;
import java.util.TreeMap;

public class Main {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		TreeMap<Long, Long> map = new TreeMap<>();
		int n = scan.nextInt();
		for (int i = 0; i < n; i++) {
			long a = scan.nextLong();
			if (!map.containsKey(a)) {
				map.put(a, Long.valueOf(1));
			} else {
				long temp = map.get(a);
				map.put(a, temp + 1);
			}
		}
		long sq = 1;
		int count = 0;
		int len = map.size();
		for (int i = 0; i < len; i++) {
			Entry<Long, Long> ans = map.pollLastEntry();
			long tempV = ans.getValue();
			long tempK = ans.getKey();
			for (int j = 0; j < 2; j++) {
				if (tempV >= 2) {
					tempV = tempV - 2;
					sq = sq * tempK;
					count++;
				}
			}
		}
		if (count < 2) {
			System.out.println(0);
		} else {
			System.out.println(sq);
		}

	}
}