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.Scanner;
import java.util.Arrays;
import java.util.*; 
import java.math.*;
import java.io.*;
public class Main {
 public static void main ( String args[] ) {
    Scanner s = new Scanner(System.in);
    long integer = s.nextLong();
System.out.print(integer%2 == 0 ? integer/2 : -((integer+1)/2) );

}
}





