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 n = int.Parse(Console.ReadLine());
            var an = Console.ReadLine().Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).ToList();
            var max = an.Max();
            var dic = new Dictionary<int, int>();

            foreach (var item in an)
            {
                if (dic.ContainsKey(item))
                    dic[item]++;
                else
                    dic.Add(item, 1);
            }
            var data = dic.Where(x => x.Value >= 2).OrderByDescending(x => x.Key).ToList();
            try
            {
                if (data[0].Value >= 4)
                    Console.WriteLine(data[0].Key * data[0].Key);
                else
                    Console.WriteLine(data[0].Key * data[1].Key);
            }
            catch
            {
                Console.WriteLine(0);
            }
            Console.ReadLine();
        }
    }
}