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: Wrong Answer)

using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCorder
{
    class Program
    {
        static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }
        public void Solve(ConsoleInput cin)
        {
            var N = cin.ReadInt;
            var dic = new Dictionary<int, int>();
            for(int i = 0; i < N; i++)
            {
                var A = cin.ReadInt;
                if (!dic.ContainsKey(A))
                {
                    dic.Add(A, 1);
                }
                else
                {
                    dic[A]++;
                }
            }
            dic = dic.ToDictionary(a => a.Key, a => a.Value / 2);
            var D = dic.Where(a => a.Value > 0).OrderByDescending(a => a.Key).ToDictionary(a => a.Key, a => a.Value);
            if(D.Values.Sum() <= 1)
            {
                WriteLine(0);
                return;
            }
            var K = D.Keys.ToArray();
            if(dic[K[0]] == 1)
            {
                WriteLine(K[0] * K[1]);
                return;
            }
            else
            {
                WriteLine(K[0] * K[0]);
                return;
            }
        }

        /*public long C(long n, long r)
        {
            if (r == 0 || r == n) return 1L;
            if (n < r) return 0L;
            long[,] Pascal = new long[n + 1, n + 1];
            Pascal[0, 0] = 1;
            for (long i = 1; i <= n; i++)
            {
                Pascal[i, 0] = 1;
                Pascal[i, i] = 1;
                for (long j = 1; j < n; j++)
                {
                    Pascal[i, j] = Pascal[i - 1, j] + Pascal[i - 1, j - 1];
                }
            }
            return Pascal[n, r];
        }*/

        public class ConsoleInput
        {
            private readonly System.IO.TextReader _stream;
            private char _separator = ' ';
            private Queue<string> inputStream;
            public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
            {
                this._separator = separator;
                this._stream = stream;
                inputStream = new Queue<string>();
            }
            public string Read
            {
                get
                {
                    if (inputStream.Count != 0) return inputStream.Dequeue();
                    string[] tmp = _stream.ReadLine().Split(_separator);
                    for (int i = 0; i < tmp.Length; ++i)
                        inputStream.Enqueue(tmp[i]);
                    return inputStream.Dequeue();
                }
            }
            public string ReadLine { get { return _stream.ReadLine(); } }
            public int ReadInt { get { return int.Parse(Read); } }
            public long ReadLong { get { return long.Parse(Read); } }
            public double ReadDouble { get { return double.Parse(Read); } }
            public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
            public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
            public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
        }
    }
}