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.Linq;
using System.Collections.Generic;

namespace Algorithm
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = int.Parse(Console.ReadLine());
            var A = Console.ReadLine().Split().Select(int.Parse).ToArray();
            var dict = new Dictionary<long, long>();

            foreach (var a in A)
            {
                if (dict.ContainsKey(a))
                    dict[a]++;
                else
                    dict[a] = 1;
            }

            var count = 0;
            var firstVal = 0L;
            foreach (var d in dict.OrderByDescending(z => z.Key))
            {
                if (d.Value >= 2)
                {
                    count++;
                    if (count == 1)
                        firstVal = d.Key;
                    else
                    {
                        Console.WriteLine(d.Key * firstVal);
                        return;
                    }
                }
                if (d.Value >= 4)
                {
                    Console.WriteLine(d.Key * d.Key);
                    return;
                }
            }
            Console.WriteLine(0);
        }
    }
}