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

# utils

def read_int_array(string):
	return map(lambda x: int(x), string.strip().split(' '))

# input
source = fileinput.input()
# source = open('data.txt', 'r').readlines()

data = int(source[0].strip()) + 1

if data % 2 == 0:
	print - data / 2
else:
	print data / 2
