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 (GCC 5.4.1) (judge verdict: Wrong Answer)

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

int compare_int(const void *a, const void *b)
{
	return *(long int*)b - *(long int*)a;
}

int main(void)
{
	int i, n, choice = 2;
	long int a[100000] = {};
	long int buff, max = 0;
	int s = 1;

	scanf("%d", &n);

	for (i = 0; i < n; i++) {
		scanf("%ld", &a[i]);
	}

	qsort(a, n, sizeof(long int), compare_int);

	for (i = 0; i < n - 1 && choice != 0; i++) {
		if (a[i] == a[i + 1]) {
			s *= a[i];
			i++;
			if (--choice == 0) {
				printf("%d", s);
			}
		}
	}
	printf("0");
	return 0;
}