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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;

public class Main {

        public static void main(String[]args) throws IOException {
            BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
            long number= (Long.parseLong(input.readLine()));
            long total=0;
            if (number%2==0){
                total=number/2;
            }
            else {
                total=0-(number/2+1);
            }
            System.out.println(total);
        }
}