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.*;
import java.util.*;


public class A{
	public static void main(String[] args)throws IOException{
		BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
		long num=Long.parseLong(in.readLine());
		int flag=1;
		if(num%2==1)flag=-1;
		System.out.println(flag*(num+1)/2);

	}
}