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 fileinput

input = []

for line in fileinput.input():
	try:
		input.append(int(line))
	except ValueError:
		pass

def calc(x):
	r = ((x + 1) / 2)
	if x & 1:
		r *= -1
	return r

for x in input:
	print calc(x)
