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


public class C
{
	public static void Main()
	{
		int N = Int32.Parse(Console.ReadLine());
		List<long> A = new List<long>(N);
		string[] args = Console.ReadLine().Split(' ');
		for(int i = 0; i < N; i++)
			A.Add(Int32.Parse(args[i]));
		A.Sort();
		A.Reverse();

		long edge1 = 0;
		long edge2 = 0;
		for(int i = 0; i < N-1; i++)
		{
			if(A[i] != A[i+1])
				continue;
			if(edge1 == 0)
			{
				edge1 = A[i];
				i++;  // total: i += 2
			}
			else
			{
				edge2 = A[i];
				break;
			}
		}
		
		Console.WriteLine(edge1 * edge2);
	}
}