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.IO;
using System.Linq;
using System.Runtime.ExceptionServices;
using System.Runtime.InteropServices;
using System.Text;

namespace AtCoder
{
	class Program
	{
		static void Main(string[] args)
		{
			Question.Exec();
		}
	}

	public static class Question
	{
		public static void Exec()
		{
			long n = int.Parse(Console.ReadLine());
			var a = Console.ReadLine().Split(' ').Select(i => long.Parse(i)).ToArray();
			long area = 0;

			var dict = new Dictionary<long, int>();
			foreach (var item in a) {
				if (dict.ContainsKey(item) == false) {
					dict.Add(item, 1);
				} else {
					++dict[item];
				}
			}

			var list = new List<long>();
			foreach (var item in dict) {
				if (item.Value >= 2) {
					list.Add(item.Key);
				}
			}

			if (list.Count >= 2) {
				for (var i = 0; i < list.Count - 1; i = i + 2) {
					area += list[i] * list[i + 1];
				}
			}

			Console.WriteLine($"{area}");

			Console.ReadKey();
		}
	}
}