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 ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var dic = new Dictionary<int, int>();

            Console.ReadLine();
            Console.ReadLine()
                .Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)
                .Select(x => int.Parse(x))
                .ToList()
                .ForEach(x => 
                {
                    if (!dic.ContainsKey(x))
                        dic.Add(x, 1);
                    else
                        dic[x] += 1;
                });

            var orderedDic = new Dictionary<int, int>();
            dic.Where(x => x.Value >= 2)
                .OrderByDescending(x => x.Key)
                .ToList()
                .ForEach(x =>
                {
                    orderedDic.Add(x.Key, x.Value);
                });

            if (orderedDic.Count < 1)
            {
                Console.WriteLine(0);
                return;
            }
            var el1 = orderedDic.First(x => true);
            if (el1.Value >= 4)
            {
                Console.WriteLine(el1.Key * el1.Key);
                return;
            }
            orderedDic.Remove(el1.Key);
            var el2 = orderedDic.First(x => true);
            if (orderedDic.Count +1  <2)
            {
                Console.WriteLine(0);
                return;
            }
            Console.WriteLine(el2.Key*el1.Key);


        }
    }
}