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.Text;
using System.Threading.Tasks;

namespace ABC_C
{
    class C071
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] s = Console.ReadLine().Split(' ');
            Dictionary<int, int> dic = new Dictionary<int, int>();
            foreach(string a in s)
            {
                int i = int.Parse(a);
                if (!dic.ContainsKey(i)) dic.Add(i, 0);
                dic[i]++;
            }

            var v = dic.Where(x => x.Value >= 2).OrderByDescending(x => x.Key).ToList();
            long Srec=0;
            if (v.Count == 0)
            {
                Srec = 0;
            }
            else if (v[0].Value >= 4)
            {
                Srec = v[0].Key * v[0].Key;
            }
            else if(v.Count==1)
            {
                Srec = 0;
            }
            else
            {
                Srec = v[0].Key * v[1].Key;
            }
            Console.WriteLine(Srec);
        }
    }
}