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

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            int[] A = Array.ConvertAll(Console.ReadLine().Split(' '), new Converter<string, int>(x => int.Parse(x)));
            Array.Sort(A);
            A.Reverse();
            long rect_upper = 0;
            long rect = 0;
            for (int i = 1; i < A.Length; i++)
            {
                if (rect_upper == 0)
                {
                    if (A[i - 1] == A[i])
                    {
                        rect_upper = A[i];
                        i++;
                    }
                }
                else
                {
                    if (A[i - 1] == A[i])
                    {
                        rect= A[i] * rect_upper;
                    }
                }

            }

            Console.WriteLine(rect);
            Console.ReadLine();
        
        }
    }
}