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-csc 3.5.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using static System.Console;
using static System.Math;
using static System.Array;

class Program
{
	public static void Main()
	{
		var n = int.Parse(ReadLine());
		var a = ReadLine().Split().Select(long.Parse).ToArray();
		var d = new Dictionary<long, long>();
		for(int i = 0; i < n; i++)
		{
			if(!d.ContainsKey(a[i]))
				d.Add(a[i], 0);
			d[a[i]]++;
		}
		var four = 0L;
		if(d.Where(x => x.Value >= 4).Count() > 0)
			four = d.Where(x => x.Value >= 4).Max(x => x.Key);
		var two1 = 0L;
		if(d.Where(x => x.Value >= 2).Count() > 0)
			two1 = d.Where(x => x.Value >= 2).Max(x => x.Key);
		var two2 = 0L;
		if(d.Where(x => x.Value >= 2 && x.Key != two1).Count() > 0)
			two2 = d.Where(x => x.Value >= 2 && x.Key != two1).Max(x => x.Key);
		WriteLine(Max(four * four, two1 * two2));
	}
}