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)
                .ToList()
                .ForEach(x =>
                {
                    orderedDic.Add(x.Key, x.Value);
                });
            var unko = new List<int>();
            orderedDic
                .ToList()
                .ForEach(x =>
                {
                    var value = x.Value;
                    if (value % 2 != 0)
                        value--;
                    for (int i = 0; i < value; i++)
                    {
                        unko.Add(x.Key);
                    }

                });
            if (unko.Count < 4)
            {
                Console.WriteLine(0);
                return;
            }
            unko.Sort();
            unko.Reverse();
            Console.WriteLine(unko[0]*unko[2]);
            Console.ReadKey();
            
        }
    }
}