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
from sys import stdin, stdout


def main():
    line = stdin.readline()
    parts = line.split()
    n = int(parts[0])
    ans=0


    if n%2==0:
        ans = int(n/2)
    else:
        ans = -int(n/2)-1
    stdout.write(str(ans))


main()
	     	  	  		 	  	 	 	   		