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-mcs 6.8.0.105) (judge verdict: Wrong Answer)

using System;
using System.Linq;
using System.Collections.Generic;
using System.IO;
using static System.Console;

namespace atcorder2
{
    class Program
    {
        public static long Read()
        {
            return long.Parse(ReadLine());
        }

        public static long[] Reads()
        {
            return ReadLine().Split().Select(long.Parse).ToArray();
        }


        public static long gcd(long x,long y)
        {
            if (y < x) gcd(y, x);
            while (y != 0)
            {
                var z = x % y;
                x = y;
                y = z;
            }
            return x;
        }

        public static long C(long n,long k)
        {
            long x=1;
            for (long i = 1; i <=k; i++)
            {
                x *= n - i + 1;
                x /= i;
            }
            return x;
        }
        static void Main(string[] args)
        {
            var sw = new StreamWriter(OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);

            var n = Read();
            var a = Reads();
            var d = new Dictionary<long, long>();
            for (int i = 0; i < n; i++)
            {
                if (d.ContainsKey(a[i])) d[a[i]]++;
                else d.Add(a[i], 1);
            }
            long ans = 0;
            var f = false;
            var nd = d.OrderByDescending(x => x.Key);
            foreach (var item in nd)
            {
                if (item.Value >= 4)
                {
                    ans = item.Key * item.Key;
                    break;
                }
                else if (item.Value >= 2)
                {
                    if (f)
                    {
                        ans *= item.Key;
                        break;
                    }
                    else
                    {
                        ans = item.Key;
                        f = true;
                    }
                }
            }
            WriteLine(ans);

            Console.Out.Flush();
        }
    }
}