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
//package codeforces;

import java.math.BigInteger;
import java.util.Scanner;

public class Calc {

    public static void main(String[] args) {
        Scanner in = new Scanner (System.in);
         long n = in.nextLong();
        long t;
        if(n % 2 ==0){
            System.out.println(""+(n/2));
        }
        else{
            t = n/2;
            System.out.println(""+(t -n));
        }
        

    }

}
