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


    }
}