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.Text;
using System.Numerics;
using System.IO;

namespace SolutionCS
{
    class Program
    {
        class CountedHashSet
        {

            class InveerseCompare : IComparer<int>
            {
                public int Compare(int x, int y)
                {
                    return y - x;
                }
            }

            public class Counter
            {
                public Counter()
                {
                    count = 1;
                }

                public int count { get; set; }
            }


            public SortedDictionary<int, Counter> keySet;

            public CountedHashSet()
            {
                keySet = new SortedDictionary<int, Counter>(new InveerseCompare());
            }

            public void Add(int key)
            {
                Counter c;

                if (keySet.TryGetValue(key, out c))
                {
                    c.count++;
                }
                else
                {
                    keySet.Add(key, new Counter());
                }
            }

            public int GetKeyCount(int key)
            {
                Counter c;

                if (keySet.TryGetValue(key, out c))
                {
                    return c.count;
                }

                return 0;
            }
        }

        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            CountedHashSet hs = new CountedHashSet();

            string[] s = Console.ReadLine().Split(' ');
            for (int i = 0; i < N; i++)
            {
                int v = int.Parse(s[i]);
                hs.Add(v);
            }

            int ans = 0;
            foreach (KeyValuePair<int, CountedHashSet.Counter> e in hs.keySet)
            {
                if (e.Value.count >= 2)
                {
                    if ( e.Value.count >= 4)
                    {
                        if ( 0 == ans)
                        {
                            Console.WriteLine(Math.BigMul(e.Key, e.Key));
                        }
                        else
                        {
                            Console.WriteLine(Math.BigMul(ans, e.Key));
                        }
                        return;
                    }

                    if (0 == ans)
                    {
                        ans = e.Key;
                    }
                    else
                    {
                        Console.WriteLine(Math.BigMul(ans, e.Key));
                        return;
                    }
                }
            }

            Console.WriteLine(0);
        }
    }
}