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: Accepted)

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

int compar_int(int *a, int *b) {
	if (*a > *b) return 1;
	if (*a == *b) return 0;
	return -1;
}

int main() {
	long long int ans;
	int l[2];
	l[0] = 0; l[1] = 0;
	int N;
	scanf("%d", &N);
	int A[N];
	for (int i = 0; i < N; i++) scanf("%d", A+i);
	
	qsort(A, N, sizeof(int), compar_int);

	for (int i = N-1; i > 0 && !l[1]; i--) {
		if (A[i] == A[i-1]) {
			if (!l[0]) l[0] = A[i];
			else l[1] = A[i];
			i--;
		}
	}

	ans = (long long int)l[0] * l[1];

	printf("%lld\n", ans);

	return 0;	
}