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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System.Collections.Generic;
using System.Linq;
using System;
class Program
{
    static void Main()
    {
        int stickCount = int.Parse(Console.ReadLine());
        string line = Console.ReadLine();
        string[] strs = line.Split(' ');
        int[] nums = new int[stickCount];
        for (int i = 0; i < stickCount; i++)
        {
            nums[i] = int.Parse(strs[i]);
        }
        Array.Sort(nums);
        Array.Reverse(nums);

        int[] biggestNumber = new int[2];
        int bigCount = 0;
        for (int i = 0; i < stickCount - 1; i++)
        {
            if(bigCount == 0)
            {
                if (nums[i] == nums[i + 1])
                {
                    biggestNumber[0] = nums[i];
                    bigCount++;
                }
            }
            else
            {
                if (biggestNumber[0] != nums[i] && nums[i] == nums[i + 1])
                {
                    biggestNumber[1] = nums[i];
                    break;
                }
            }
        }

        int result = biggestNumber[0] * biggestNumber[1];
        Console.WriteLine(result.ToString());
    }
}