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; 
class Program
    {
        static void Main(string[] args)
        {
            //4<=N<=10^5
            //1<=A<=10^9
            int N = int.Parse(Console.ReadLine());
            var Array = Console.ReadLine().Split().Select(x => int.Parse(x)).ToArray();
            var Dis = Array.Distinct().ToList();
            Dis.Sort((a, b) => b - a);
            var NumCount = new Dictionary<int, int>();
            int Num = 0;
            for (int i = 0; i < Dis.Count(); i++)
            {
                Num = Array.Count(x => x == Dis[i]);
                if (Num > 1)
                    NumCount.Add(Dis[i], Num);

            }

            if (NumCount.Count < 2)
            {
                Console.WriteLine("0"); return;
            }

            var First = NumCount.Keys.First();
            var Count = NumCount.Values.First();
            if (Count > 3)
            {
                Console.WriteLine(First * First);
            }
            else
            {
                var Second = NumCount.Keys.Where(x => x < First).First();
                Console.WriteLine(First * Second);
            }

        }
    }