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

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        Map<Long, Long> map = new TreeMap<>(Comparator.reverseOrder());
        for (int i = 0; i < n; i++) {
            long a = Long.parseLong(sc.next());
            map.merge(a, 1L, Long::sum);
        }
        List<Long> list2 = new ArrayList<>();
        List<Long> list4 = new ArrayList<>();
        for (Map.Entry<Long, Long> m : map.entrySet()) {
            if (m.getValue() >= 4) {
                list4.add(m.getKey());
            }
            if (m.getValue() >= 2) {
                list2.add(m.getKey());
                if (list2.size() == 2) {
                    break;
                }
            }
        }
        if (list4.size() != 0) {
            System.out.println(list4.get(0) * list4.get(0));
        } else if (list2.size() < 2) {
            System.out.println(0);
        } else {
            System.out.println(list2.get(0) * list2.get(1));
        }
    }
}