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 Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.util.Scanner;

/**
 *
 * @author Owner
 */
public class Main{

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int[] a=new int[n];
        int b=-1,t=-1;
        
        for(int i=0;i<n;i++){
            a[i]=sc.nextInt();
        }
        a=Bubble_sort(a,n);
        for(int i=1;i<n;i++){
            if(a[i]==a[i-1]&&b==-1){
                b=a[i];
                i++;
            }
            else if(a[i]==a[i-1]&&t==-1){
                t=a[i];
                break;
            }
        }
        if(t!=-1){
            System.out.println(t*b);
        }
        else{
            System.out.println("0");
        }
    }
    static int[] Bubble_sort(int a[],int n){	//バブルソート
        int i,j;
        
        for(i=0;i<n-1;i++){
            for(j=n-1;j>i;j--){
                if(a[j]>a[j-1]){
                    int tmp=a[j];
                    a[j]=a[j-1];
                    a[j-1]=tmp;
                }
            }
        }
        return a;
    }
}