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());
            string[] inputStr = Console.ReadLine().Split(' ');
            int len = inputStr.Length;
            long[] A = new long[len];
            for(int i=0; i<len; i++){
                A[i] = long.Parse(inputStr[i]);
            }
     
            /*
            bool sorted = false;
            int x;
            while(!sorted){
                sorted = true;
                for(int i=0; i<len-1; i++){
                    if(input[i]<input[i+1]){
                        x = input[i];
                        input[i] = input[i+1];
                        input[i+1] = x;
     
                        sorted = false;
                    }
                }
            }
            */
     
            Array.Sort(A);
            Array.Reverse(A);
     
            long[] edges = {0,0};
            int j = 0;
            for(int i=0; i<len-1; i++){
                if(A[i]==A[i+1]){
                    edges[j] = A[i];
                    j++;
                    i++;
                }
                if(j==2) break;
            }
     
            Console.WriteLine(edges[0]*edges[1]);
        }
    }