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>

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

int main() {
	int	N;
	int a = -1, b = -1;
	int ans = 0;
	scanf("%d", &N);
	int *A = (int *)malloc(sizeof(int)*N);
	for (int i = 0; i < N; i++)
	{
		scanf("%d", &A[i]);
	}
	qsort(A, N, sizeof(int), compare_int);
	//for (int i = 0; i < N; i++)
	//{
	//	printf("%d ", A[i]);
	//}
	for (int i = 1; i < N; i++)
	{
		if (a == -1 && A[i] == A[i - 1])
		{
			a = A[i];
			i++;
			continue;
		}
		if (a != -1 && b == -1 && A[i] == A[i - 1])
		{
			b = A[i];
			break;
		}
	}
	if (a != -1 && b != -1)
	{
		ans = a * b;
	}
	printf("%d\n", ans);
	return 0;
}