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: Runtime Error)

package atCoder.ABC071;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        List<Integer> list = new ArrayList<Integer>();
        for (int i = 0 ; i < n ; i++) {
            list.add(sc.nextInt());
        }
        list.sort(Comparator.reverseOrder());

        int longSide = 0;
        int shortSide = 0;
        //降順にならべてあるリストから長辺と短辺をとっていく。長辺と短辺の長さが同じになることもある。
        for (int i = 0 ; i < n - 1 ; i++) {
            if (list.get(i).equals(list.get(i + 1))  && longSide == 0) {
                longSide = list.get(i);
                i++;
            } else if (list.get(i).equals(list.get(i + 1))) {
                shortSide = list.get(i);
                break;
            }
        }
        System.out.println((long) longSide * shortSide);

    }

}