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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.util.*;

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

        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < n; i++) {
            int a = sc.nextInt();
            int count = map.getOrDefault(a, 0);
            map.put(a, ++count);
        }

        List<Integer> candidate = new ArrayList<>();
        for (Map.Entry<Integer, Integer> e : map.entrySet()) {
            if (e.getValue() >=4) {
                candidate.add(e.getKey());
                candidate.add(e.getKey());
            } else if (e.getValue() >= 2) {
                candidate.add(e.getKey());
            }
        }

        candidate.sort(Collections.reverseOrder());
        if (candidate.size() < 2) {
            System.out.println(0);
        } else {
            System.out.println((long)candidate.get(0) * candidate.get(1));
        }
    }
}