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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class Program
{
    private int N;
    private int[]A;
    private void Scan()
    {
        N=int.Parse(Console.ReadLine());
        A=new int[N];
        var l=Console.ReadLine().Split(' ');
        for(int i=0;i<N;i++)
        {
            {
                A[i]=int.Parse(l[i]);
            }
        }
    }
    
    
    
    public void Solve()
    {
        Scan();
        Array.Sort(A,(a,b)=>-a.CompareTo(b));
        int q=0;
        int cnt=0;
        int[] ans=new int[2];
        int j=0;
        foreach(int i in A)
        {
            if(q!=i)
            {
                q=i;
                cnt=1;
            }
            else
            {
                cnt++;
                if(cnt==2)
                {
                    if(j<2)
                    {
                        ans[j]=i;
                        j++;
                    }
                 
                    cnt=0;   
                }
            }
        }
        Console.WriteLine((long)ans[0]*ans[1]);
        
    } 
    
    static void Main()
    {
        new Program().Solve();
    }
}