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: Accepted)

using System;
using System.Linq;

namespace arc081_a
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            long[] x = Console.ReadLine().Split().Select(long.Parse).ToArray();
            Array.Sort(x); Array.Reverse(x);
            long han = 0; bool find = false;
            for (int i = 0; i < n && !find; i++)
            {
                if (i <= n - 4)
                {
                    if (han == 0 && x[i] == x[i + 1] && x[i] == x[i + 2] && x[i] == x[i + 3])
                    {
                        find = true;
                        Console.WriteLine(x[i] * x[i]);
                    }
                    else if (x[i] == x[i + 1])
                    {
                        if (han != 0)
                        {
                            find = true;
                            Console.WriteLine(han * x[i]);
                        }
                        else
                        {
                            han = x[i]; i++;
                        }
                    }
                }
                else if (i <= n - 2)
                {
                    if (x[i] == x[i + 1])
                    {
                        if (han != 0)
                        {
                            find = true;
                            Console.WriteLine(han * x[i]);
                        }
                        else
                        {
                            han = x[i]; i++;
                        }
                    }
                }
            }
            if (!find) Console.WriteLine(0);
        }
    }
}