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 fir
{
 public static void main(String args[])
 {
 Scanner fo=new Scanner(System.in);

 
 long n=fo.nextLong();
long sum;
if(n%2==0)
sum=n/2;
else
sum=-(n+1)/2;

System.out.println(sum);
}   
}