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: Runtime Error)

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>



int main()
{
	int i, N;

	scanf("%d", &N);

	int number[11];
	int t[11];

	for (i = 0; i < N; i++) scanf("%d", &number[i]);

	int  j, tmp;
	
	
	
	
	for (i = 0; i < N; ++i) {
		for (j = i + 1; j < N; ++j) {
			if (number[i] > number[j]) {
				tmp = number[i];
				number[i] = number[j];
				number[j] = tmp;
			}
		}
	}

	
	int Ans = 1;
	int s = 0;

	for (i = N-1; i>0; i--) {
		if (number[i] ==number[i-1] && s<2 ) {
			Ans *= number[i];
			s++;
			i--;
		}
	}

	

	if (s== 2) {
		printf("%d", Ans);
	}
	else printf("-1");
}