For a positive integer n let's define a function f:

f(n) = - 1 + 2 - 3 + .. + ( - 1)nn

Your task is to calculate f(n) for a given integer n.

Input

The single line contains the positive integer n (1 ≤ n ≤ 1015).

Output

Print f(n) in a single line.

Examples

Input

4


Output

2


Input

5


Output

-3

Note

f(4) = - 1 + 2 - 3 + 4 = 2

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3
import java.util.*;
 
public class Main
{
    public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		long n = scan.nextLong();
		long a = n/2;
		long sum_even = a*(a+1);
		
		long k =0;
		if(n%2==0){
		     k = n/2;
		}
		else{
		    k = n/2+1;
		}
		
		long sum_odd = k*k;
		System.out.println(sum_even-sum_odd);
		
		
		    
		
	}
}