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.Diagnostics;
using System.IO;

class _Class
{
	void _Do()
	{
		//
		var _s = _stringArray();
		var n_ = int.Parse(_s[0]);

		var a_ = _intArray();

		Array.Sort(a_);
		Array.Reverse(a_);

		int a=0;
		int b = 0;
		int i = 0;
		for (;i<n_-1;i++) {

			int x1 = a_[i];
			int x2 = a_[i+1];

			if (x1 == x2)
			{
				if (a == 0)
				{
					a = x1;
					i++;
				}
				else
				{
					b = x1;
					break;
				}
			}
		}
		Console.WriteLine(a*b);
	}	

	static void Main(string[] args)
	{
		new _Class()._Do();
	}


	// 1
	string[] _stringArray()
	{
		return Console.ReadLine().Split();
	}

	// 2.1 -2,147,483,648 ~ +2,147,483,647
	int[] _intArray()
	{
		var _s = Console.ReadLine().Split();

		var x = new int[_s.Length];
		for (var i = 0; i < _s.Length; i++)
			x[i] = int.Parse(_s[i]);

		return x;
	}

	// 2.2 -9,223,372,036,854,775,808 ~ +9,223,372,036,854,775,807
	long[] _longArray()
	{
		var _s = Console.ReadLine().Split();

		var x = new long[_s.Length];
		for (var i = 0; i < _s.Length; i++)
			x[i] = long.Parse(_s[i]);

		return x;
	}
}