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.*;
/**
 * Created by AdminPC on 11.02.2017.
 */
public class test {
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        long n = in.nextLong();
        long c=0;
        if (n%2==0){
            c = n/2;
            System.out.println(c);
        }
        if (n%2==1) {
            n++;
            c=n/2;
            c=-1*c;
            System.out.println(c);
        }



    }
}
//-1+2-3+4-5+6-7+8-9+10-11+12