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 abc71
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            int[] A = new int[N];
            long[] E = new long[2];
            E[0] = 0;
            E[1] = 0;
            int f = 0;
            string[] str = Console.ReadLine().Split(' ');
            for (int i = 0; i < N; i++)
            {
                A[i] = int.Parse(str[i]);

            }
            Array.Sort(A);
            for(int i = N - 1; i >= 1; i--)
            {
                if (A[i] == A[i - 1])
                {
                    E[f] = (long)A[i];
                    f++;
                    i--;
                }
                if (f == 2)
                {
                    break;
                }
            }
            Console.WriteLine((long)E[0] * E[1]);
        }
    }
}