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>
#include <string.h>

int _compare(const void *a, const void *b) {
	if(*(int *)a < *(int *)b)
		return -1;
	else if(*(int *)a == *(int *)b)
		return 0;
	else
		return 1;
}

void getInput(int *data, long long int data_cnt, long long int str_size) {
	long long int i, N;
	char str[str_size], *tok;

	i = 0;
	N = data_cnt;
	gets(str);
	tok = strtok(str, " ");
	data[i++] = atoi(tok);
	for(; i < N; i++) {
		tok = strtok(NULL, " ");
		if(tok == NULL)
			break;
		data[i] = atoi(tok);
	}
}

int main(void) {
	int d[100000];
	long long int num, i, l1 = 0, l2 = 0;

	scanf("%lld", &num);
	getchar();
	getInput(d, num, 10 * 200000);

	qsort(d, num, sizeof *d, _compare);
	for(i = num - 1; i >= 1; i--) {
		if(d[i] == d[i - 1]) {
			if(l1 == 0 && l2 == 0)
				l1 = d[i--];
			else if(l2 == 0) {
				l2 = d[i--];
				break;
			}
		}
	}

	if(l2 == 0)
		printf("0");
	else
		printf("%lld", l1 * l2);

	return 0;
}