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.Generic;
using System.IO;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;

namespace AtCoder
{
	public class Program
	{
		static void Main()
		{
			int n = int.Parse(Console.ReadLine());
			var a = Console.ReadLine().Split(' ').Select(long.Parse).ToArray();

			var dict = new Dictionary<long, int>();
			for (int i = 0; i < n; i++) {
				if (dict.ContainsKey(a[i])) {
					dict[a[i]]++;
				} else {
					dict[a[i]] = 1;
				}
			}

			var a2 = new List<long>(n);
			var a4 = new List<long>(n);
			foreach (var key in dict.Keys) {
				if (dict[key] >= 2) {
					a2.Add(key);
				}

				if (dict[key] >= 4) {
					a4.Add(key);
				}
			}

			var as2 = a2.OrderByDescending(x => x).ToArray();
			var as4 = a4.OrderByDescending(x => x).ToArray();

			long ans1 = 0;
			if (as2.Length >= 2) {
				ans1 = as2[0] * as2[1];
			}

			long ans2 = 0;
			if (as4.Length >= 1) {
				ans2 = as4[0] * as4[0];
			}

			long ans = Math.Max(ans1, ans2);
			Console.WriteLine(ans);
		}
	}
}