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.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp15
{
    class Class51
    {
        static void Main()
        {
            //入力
            int n = int.Parse(Console.ReadLine());
            var a = Console.ReadLine().Split().Select(long.Parse).ToArray();

            //計算
            Array.Sort(a);
            long[] pair = new long[2];
            for(int i = n - 2; i >= 0; i--)
            {
                if (a[i] == a[i + 1])
                {
                    if (pair[0] == 0)
                    {
                        pair[0] = a[i];
                        i--;
                    }
                    else if (pair[1] == 0)
                    {
                        pair[1] = a[i];
                        break;
                    }
                }
            }

            //出力
            Console.WriteLine(pair[0] * pair[1]);
        }
    }
}