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.*;

public class Main {
    static int n;
    static Integer[] dp;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        n = sc.nextInt();
        dp = new Integer[n];

        for (int i = 0; i < n; i++) {
            dp[i] = sc.nextInt();
        }
        Arrays.sort(dp, Collections.reverseOrder());
        int h = 0;
        int w = 0;
        int tmp = dp[0];
        for (int i = 1; i < n; i++) {
            if (tmp == dp[i]) {
                if (h == 0) {
                    h = tmp;
                } else {
                    w = tmp;
                    break;
                }
                tmp = -1;
            } else {
                tmp = dp[i];
            }
        }
        System.out.println(h*w);
    }
}