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;

class MakeARectangle
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine()); // 扱うデータの個数
        var stick = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToList();

        var result = new List<int>();
        var unique = new List<int>();

        foreach(var x in stick)
        {
            if (!result.Contains(x))
            {
                if (unique.Contains(x))
                {
                    result.Add(x);
                }
                else
                {
                    unique.Add(x);
                }
            }
        }

        result.OrderByDescending(x => x);
        Console.WriteLine($"{result.FirstOrDefault() * result.Skip(1).FirstOrDefault()}");

    }
}