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 Function {
	
	public static long func_real(long n) {
		long out = -1;
		for(long l = 2; l <= n; l++) {
			int sign = out > 0 ? 1 : -1;
			out += l * -sign;
		}
		return out;
	}
	
	public static long func(long n) {
		long out = -(n + 1) / 2;
		if(((n + 1) % 2) == 1) out *= -1;
		return out;
	}
	
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		long l = in.nextLong();
		System.out.println(func(l));
	}
}
