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 __future__ import division
from collections import Counter as ctr
import math
def rl():
	return [int(i) for i in raw_input().split()]

def rm(n=None):
	if n is None:
		n = input()
	return [raw_input() for i in range(n)]

def rlm(n=None):
	if n is None:
		n = input()
	return [rl() for i in range(n)]

def yn(b):
	if b:
		print "YES"
	else:
		print "NO"

n = input()
if n % 2 == 0:
	print n//2
else:
	print -(n+1)//2