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;
using System.Linq;
using System.Collections.Generic;

class Cmondai
{
    static void Main()
    {
        var N = Int32.Parse(Console.ReadLine());
        var A = Console.ReadLine().Split(' ').Select(x => Int32.Parse(x)).OrderBy(x=>x).ToArray();
        var max = 0;
        var max2 = 0;
        var currentNum = A[0];
        var currentNumCount = 1;
        for (int i=0;i<N;++i)
        {
            if (A[i] == currentNum)
            {
                currentNumCount++;
                if (2 == currentNumCount)
                {
                    if (max < currentNum)
                    {
                        max2 = max;
                        max = currentNum;
                    }else if (max2<currentNum)
                    {
                        max2 = currentNum;
                    }
                }else if(4 == currentNumCount)
                {
                    if (max < currentNum)
                    {
                        max2 = max;
                        max = currentNum;
                    }
                    if (max2 < currentNum)
                    {
                        max2 = currentNum;
                    }
                }
            }
            else
            {
                currentNum = A[i];
                currentNumCount = 1;
            }
        }
        var ans = max * max2;
        Console.WriteLine(ans);
    }
}