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;
using System.Threading.Tasks;

namespace ConsoleApplication8
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            string[] input = Console.ReadLine().Split(' ');
            int[] a = new int[n];
            for(int i = 0;i < n; i++)
            {
                a[i] = int.Parse(input[i]);
            }

            var dup = a
                .GroupBy(x => x)
                .Select(x => new { item = x.Key, count = x.Count() })
                .OrderByDescending(x => x.item)
                .Where(y => y.count > 1)
                .ToList();
            if (dup.Count() > 1)
            {
                Console.WriteLine(dup[0].item * dup[1].item);
            }
            else
            {
                Console.WriteLine(0);
            }
        }
    }
}