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.IO;
using System.Linq;
using System.Numerics;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;
using static System.Math;
using Debug = System.Diagnostics.Debug;


static class P
{
    static void Main()
    {
        var n = int.Parse(Console.ReadLine());
        var a = Console.ReadLine().Split().Select(long.Parse).ToArray();
        var v = a.GroupBy(x => x).Where(x => x.Count() >= 2).OrderByDescending(x => x.Key).ToArray();
        if (v.Length < 2)
        {
            Console.WriteLine(0);
            return;
        }
        if (v[0].Count() < 4)
        {
            Console.WriteLine(v[0].Key * v[1].Key);
            return;
        }
        {
            Console.WriteLine(v[0].Key * v[0].Key);
            return;
        }
    }
}