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
// http://codeforces.com/problemset/problem/486/A

import java.util.Scanner;

public class Solution {
	
	public static void main(String[] args) {
		Scanner read = new Scanner(System.in);
		
		long n = read.nextLong();
		
		if (n % 2 == 0) {
			System.out.println( n / 2 );
		}
		else {
			System.out.println( -1 * ((n+1)/2) );
		}
		
		// Close scanner
		read.close();
	}
	
}