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.Collections.Generic;
using System.Linq;
using System.IO;
using System.Globalization;
using System.Diagnostics;
using Pair = System.Collections.Generic.KeyValuePair<int, int>;
//using System.Numerics;

class Program
{
    static void Main()
    {
        Scanner cin = new Scanner();
        //Random rnd = new Random();
        //Stopwatch sw = new Stopwatch();

        // A B C
        /*
        int A = cin.Nextint;
        int B = cin.Nextint;
        int C = cin.Nextint;
        */

        // N
        // a1 a2 ... aN¥
        int N;
        int[] A;

        N = cin.Nextint;
        A = cin.Scanint;


        // N M
        // a1 b1
        // ...
        // aM bM
        /*
        int N;
        int M;
        int[] A = new int[M];
        int[] B = new int[M];

        N = cin.Nextint;
        M = cin.Nextint;

        for(int i=0; i<M; i++)
        {
            A[i] = cin.Nextint;
            B[i] = cin.Nextint;
        }
         */



        Array.Sort(A);
        Array.Reverse(A);

        int current = -1;
        int l = -1;
        int s = -1;
        foreach (int a in A)
        {
//            Console.WriteLine(String.Format("a: {0}, current: {1}", a, current));
            if (a == current)
            {
                if (l == -1)
                {
                    l = a;
//                    Console.WriteLine(l);
                    current = -1;
                }
                else
                {
                    s = a;
                    break;
                }

            } else
            {

                current = a;

            }

        }

//        Console.WriteLine(String.Format("s: {0}", s));

        long r;
        if(s == -1)
        {
            r = 0;
        }else
        {
            r = l*s;
        }
            
        /** ここに記述 */

        Console.WriteLine(r);
        //Console.WriteLine(result ? "Yes" : "No");
        Console.Out.Flush();
    }
}

class Scanner
{
    string[] s; int i;
    char[] cs = new char[] { ' ' };
    public Scanner() { s = new string[0]; i = 0; }
    public string[] Scan { get { return Console.ReadLine().Split(); } }
    public int[] Scanint { get { return Array.ConvertAll(Scan, int.Parse); } }
    public long[] Scanlong { get { return Array.ConvertAll(Scan, long.Parse); } }
    public double[] Scandouble { get { return Array.ConvertAll(Scan, double.Parse); } }
    public string Next
    {
        get
        {
            if (i < s.Length)
            {
                return s[i++];
            }
            string st = Console.ReadLine();
            while (st == "")
            {
                st = Console.ReadLine();
            }

            s 
            = st.
                Split
                (cs, StringSplitOptions.RemoveEmptyEntries);
            i = 0;
            return Next;
        }
    }
    public int Nextint { get { return int.Parse(Next); } }
    public long Nextlong { get { return long.Parse(Next); } }
    public double Nextdouble { get { return double.Parse(Next); } }
}