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.HashMap;
import java.util.Map;

public class Main {

    public static void main(String[] args) {
        int N = 10;
        int[] sticks = {3, 3, 3, 3, 4, 4, 4, 5, 5, 5}; // {3,1,2,4,2,1};

        int area = makeRect(N, sticks);
        System.out.println(area);
    }

    private static int makeRect(int n, int[] sticks) {;
        int area = 0, max1 = 0, max2 = 0, temp = 0;
        if (n < 4)
            System.out.println("insufficient stick");
        else {
            HashMap<Integer, Integer> stickPair = new HashMap<>();
            for (int i=0; i<sticks.length; i++){
                if (stickPair.containsKey(sticks[i])){
                    int val = stickPair.get(sticks[i]);
                    stickPair.put(sticks[i], val+1);
                } else
                    stickPair.put(sticks[i], 1);
            }

            for (Map.Entry map : stickPair.entrySet()){
                int val = (int) map.getValue();
                int key = (int) map.getKey();
                if ( val >= 2 ){
                    if (key > max1){
                        temp = max1;
                        max1 = key;
                    }
                    if (max1 > max2){
                        max1 = temp > max2 ? temp : max2;
                        max2 = key;
                    }
                }
            }
            area = max1 * max2;
        }
        return area;
    }
}