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;

namespace Practice4
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            int[] A = new int[N];
            int tmp = 0;
            int a = 0;
            int b = 0;
            double S = 0;
 
            String s = Console.ReadLine();
            String[] input = s.Split(' ');
 
            for(int i = 0; i < N; i++)
            {
                A[i] = int.Parse(input[i]);
            }
 
            for (int j = 0; j < N; j++)
            {
                for (int k = j + 1; k < N; k++)
                {
                    if (A[j] < A[k])
                    {
                        tmp = A[j];
                        A[j] = A[k];
                        A[k] = tmp;
                    }
                }
            }
 
            for(int l = 0; l < N-1; l++)
            {
                if (a == 0 && A[l] == A[l + 1])
                {
                    a = A[l];
                    A[l+1] = 0;
                }
 
                if(a != 0 && A[l] == A[l + 1])
                {
                    b = A[l];
                    break;
                }
            }
 
            S = a * b;
            Console.WriteLine("{0}", S);
 
        }
    }
}