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 (Clang 3.8.0) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>

int	desc(const void *a, const void *b)
{
	long *v1 = (long *)a;
	long *v2 = (long *)b;

	return (*v2 - *v1);
}

int	main(void)
{
	int		n;
	long	a[100000];
	long	side[2] = {};

	scanf("%d", &n);
	for (int i = 0; i < n; i++)
	{
		scanf("%ld", &a[i]);
	}
	qsort(a, n, sizeof(long), desc);
	for (int i = 1; i < n; i++)
	{
		if (a[i - 1] == a[i])
		{
			if (side[0] == 0)
			{
				side[0] = a[i];
			}
			else
			{
				side[1] = a[i];
				break;
			}
			i++;
		}
	}
	printf("%ld\n", side[0] * side[1]);
	return (0);
}