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: Time Limit Exceeded)

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;
        int[] input = new int[len];
        for(int i=0; i<len; i++){
            input[i] = int.Parse(inputStr[i]);
        }

        bool sorted = false;
        int x;
        //バブルソート
        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;
                }
            }
        }

        int[] edges = new int[2];
        int j = 0;
        for(int i=0; i<len-1; i++){
            if(j==2) break;

            if(input[i]==input[i+1]){
                edges[j] = input[i];
                j++;
                i++;
            }
        }

        Console.WriteLine(edges[0]*edges[1]);
    }
}