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.ArrayList;
import java.util.Collections;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Main {
    public static void main(String args[]) {
        Scanner input = new Scanner(System.in);
        long sum=0;
        long num= input.nextLong();
            if(num%2==0){
                sum=num/2;
                System.out.println(sum);
                
            }else{
                
                sum=(((num/2)+1)*-1);
                System.out.println(sum);

        }

        
    }
}
