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.Arrays;
import java.util.HashMap;
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        HashMap<Integer, Integer> map = new HashMap<>();
        int n = in.nextInt();
        for(int i = 0; i < n; i++) {
            int a = in.nextInt();
            if(map.containsKey(a)) {
                map.put(a, map.get(a) + 1);
            }else {
                map.put(a, 1);
            }
        }
        Object[] list = (Object[])(map.keySet().stream().filter(m -> map.get(m) > 1).toArray());
        Arrays.sort(list);
        if(list.length > 1) {
            int a = (Integer)(list[list.length - 1]);
            int b = (Integer)(list[list.length - 2]);
            System.out.println((long)a * (long)b);
        }else {
            System.out.println(0);
        }
    }
}