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.Linq;
using System.Collections.Generic;
class Program
{
    static void Main(string[] args)
    {
        int N = int.Parse(Console.ReadLine());
        int[] A = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        Array.Sort(A);
      
        int x = 0;
        int y = 0;
        int i = N - 1;
        for(;i > 0;i--)
        {
          if(A[i] == A[i - 1])
          {
            x = A[i];
            i -= 2;
            break;
          }
        }
        for(; i > 0;i--)
        {
          if(A[i] == A[i - 1])
          {
            y = A[i];
            break;
          }
        }
      
      Console.WriteLine((long)x*y);
    }
}