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
/*
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 * and open the template in the editor.
 */
import java.util.*;
/**
 *
 * @author Hai
 */
public class A486 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        long n = in.nextLong();
        long result = 0;
        if(n%2==0)
            result = n/2;
        else{
            result = (n+1)/2;
            result *= -1;
        }
        System.out.println(result);
    }
}
