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 sys

if __name__ == '__main__':
	n = int(raw_input())
	odd = n%2
	an = abs(n)
	if odd:
		ot = (1 + an)*(n/2+1)/2
		et = (2 + an - 1 )* (n/2)/2
	else:
                ot = (1 + an - 1)*(n/2)/2
                et = (2 + an )* (n/2)/2
	print -ot + et
