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.util.Map;


public class codef {
    public static void main(String[] args) throws IOException {
        BufferedReader B = new BufferedReader(new InputStreamReader(System.in));
        long n = Long.parseLong(B.readLine());
        if(n%2 == 0)
        {
            System.out.println(n/2);
            return;
        }
        System.out.println((n-1)/2 -n);
    }
}