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.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //[summary]C - Make a Rectangle
            int n = int.Parse(Console.ReadLine());
            var A = ReadLine();

            var sides = new Dictionary<long, int>();

            foreach(var Ai in A)
            {
                if (!sides.ContainsKey(Ai))
                {
                    sides.Add(Ai, 1);
                }
                else
                {
                    sides[Ai]++;
                }
            }

            var query = sides.OrderByDescending(x => x.Key);
            long area = 1;
            int count = 0;

            foreach(var q in query)
            {
                if (q.Value >= 4 && count == 0)
                {
                    area = q.Key * q.Key;
                    count += 2;
                }
                else if (q.Value >= 2)
                {
                    area *= q.Key;
                    count++;
                }

                if (count >= 2)
                {
                    break;
                }
            }

            if (count < 2)
            {
                area = 0;
            }

            Console.WriteLine(area);
        }

        static List<int> ReadLine()
        {
            var line = Console.ReadLine();
            var array = line.Split(' ');

            return array.Select(x => int.Parse(x)).ToList();
        }
    }
}