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-csc 3.5.0) (judge verdict: Wrong Answer)

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

class Test
{

    public static void Main()
    {
        var n = int.Parse(Console.ReadLine());
        var arr = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();

        Array.Sort(arr, (x, y) => { return y.CompareTo(x); });
        int num1 = -1;
        List<long> ans = new List<long>();
        for (int i = 1; i < arr.Length; i++)
        {
            if (arr[i] == arr[i - 1])
            {
                num1 = arr[i];
            }
            else
            {
                if (num1 != -1)
                {
                    ans.Add(num1);
                    num1 = -1;
                }
                if (ans.Count == 2)
                    break;
            }
        }
        if (num1 != -1 && ans.Count == 1)
        {
            ans.Add(num1);
        }
        if (ans.Count == 2)
            Console.Write(ans[0] * ans[1]);
        else
            Console.Write(0);
    }

}