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 a[100001] = { 0 };

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

int main(void) {
	int N;
	int x = 0, y = 0;

	scanf("%d", &N);
	for (int i = 0; i < N; i++) scanf("%d", &a[i]);
	qsort(a, N, sizeof(a[0]), sort);
	//for (int i = 0; i < N; i++) printf("%d\n", a[i]);

	for (int i = N - 1; i > 0; i--) {
		if (a[i] == a[i - 1]) {
			if (x == 0) x = a[i];
			else y = a[i];
			//printf("%d\n", i);
			i--;
		}
		if ((x != 0) && (y != 0)) break;
	}

	printf("%d\n", x*y);

	return 0;
}