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 Prob486A {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        long num = scanner.nextLong();
//        int sum = 0;
//        for(int i = 1; i <= num; i++){
//            if(i%2 == 0){
//                sum += i;
//            }else {
//                sum += i*-1;
//            }
//        }
        if(num%2==0){
            System.out.println(num/2);
        }else{
            System.out.println((num/-2)-1);
        }
    }
}
