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 {

    public static void main(String args[]) {

        // 入力
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        Integer[] a = new Integer[n];
        Arrays.setAll(a, i -> Integer.parseInt(sc.next()));

        // 主処理
        List<Integer> list = new ArrayList<>();
        PriorityQueue<Integer> side = new PriorityQueue<>(Collections.reverseOrder());
        for (int i = 0; i < a.length; i++) {
            if (list.contains(a[i])) {
                side.add(a[i]);
                list.remove(list.indexOf(a[i]));
            } else {
                list.add(a[i]);
            }
        }
        int result = 0;
        if (2 <= side.size()) {
            int x = side.poll();
            int y = side.poll();
            result = x * y;
        }

        // 出力
        System.out.println(result);
        sc.close();
    }
}