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.io.IOException;
//import java.io.BufferedReader;
//import java.io.InputStreamReader;
import java.util.Scanner;
import java.lang.Math;
import java.util.Arrays;
public class Main{
    public static void main(String[]args){
        long n = new Scanner(System.in).nextLong();
        System.out.println(n%2==0?n/2:-(n+1)/2);
            
                
}
}