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 Java7 (OpenJDK 1.7.0) (judge verdict: Wrong Answer)

import java.util.*;

public class Main {
    public static void  main (String[] args) {
        Scanner sc = new Scanner(System.in);
        
        int str = sc.nextInt();
        
        List<Integer> all = new ArrayList<Integer>();
        List<Integer> db = new ArrayList<Integer>();
        for (int i = 0; i < str; i++) {
            int lg = sc.nextInt();
            if (all.contains(lg)) {
                all.remove((Integer)lg);
                db.add(lg);
            } else {
                all.add((lg));
            }
        }
        
        if (db.size() < 2) {
            System.out.println(0);
        } else {
            Collections.sort(db);
            Collections.reverse(db);
            System.out.println(db.get(0) * db.get(1));
        }
    }
}