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: Time Limit Exceeded)

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

class Progaram
{
    static void Main(string[] args)
    {
        long w = 0;
        long h = 0;
        int k;
        var n = int.Parse(Console.ReadLine());
        var l = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).OrderByDescending(x => x).ToList();
        k = l.Count;

        while (k > 1)
        {
            long m = l[0];
            l.RemoveAt(0);
            k--;
            if (l[0] == m)
            {
                if (w == 0) w = m;
                else
                {
                    h = m;
                    break;
                }
                l.RemoveAt(0);
                k--;
            }
        }
        Console.WriteLine(w * h);
    }
}