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: Time Limit Exceeded)

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

int main() {
	int i, j, k=0, n, flag1, a[100000] = { 0 }, max1 = 0, max2 = 0, a2[100000][2] = { 0 }, cnt=0;

	scanf("%d", &n);
	for (i = 0; i < n; i++) {
		scanf(" %d", &a[i]);
	}
	
	for (i = 0; i < n; i++) {
		//printf("%d ", a[i]);
		for (j = 0; j < n; j++) {
			if (i == j) continue;
			if (a[i] == a[j]) {
				flag1 = 0;

				for (k = 0; k < cnt; k++) {
					if (a2[k][0] == a[i]) {
						a2[k][1]++;
						flag1 = 1;
					}
				}

				if (flag1 == 0) {

					a2[cnt][0] = a[i];
					a2[cnt][1]++;
					cnt++;
				}



			}

			
		}
	}

	for (i = 0; i < cnt; i++) {
		if (a2[i][1] >= 2) {
			if (max1 < a2[i][0] || max2 < a2[i][0]) {
				if (max1 > max2) {
					max2 = a2[i][0];
				}
				else  {
					max1 = a2[i][0];
				}
			}
		}
	}


	printf("%d\n", max1 * max2);

	//getchar();
	//getchar();
	return 0;
}