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 atcoder
{
	class Program
	{
		static void Main(string[] args)
		{
			Console.ReadLine();
			var input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToList();
			input.Sort();
			List<int> rec = new List<int>();
			for (int i = input.Count - 1; i > 0; i--)
			{
				if (input[i] == input[i - 1])
				{
					rec.Add(input[i]);
					i--;
				}
			}
			if (rec.Count < 2) Console.WriteLine(0);
			else Console.WriteLine(rec[0] * rec[1]);
		}
	}
}