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: Runtime Error)

import java.util.*;
class Main
{
	public static void main(String[] args)
	{
		Scanner sc=new Scanner(System.in);
		int num=sc.nextInt();
		int[] box=new int[num];
		for(int i=0;i<num;i++)
		{
			box[i]=sc.nextInt();
		}
		Arrays.sort(box);
		int[] numBox=new int[box[num-1]+1];
		for(int i=0;i<num;i++)
		{
			for(int j=1;j<box[num-1]+1;j++)
			{
				if(box[i]==j)
				{
					numBox[j]++;
				}
			}
		}
		int count=0,
			all=1;
		for(int i=box[num-1];i>0;i--)
		{
			if(numBox[i]>=2)
			{
				all*=i;
				count++;
			}
			if(count==2)
			{
				System.out.println(all);
				break;
			}
			if(i==1)
			{
				System.out.println("0");
				break;
			}				
		}
	}
}