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
from math import ceil, log, factorial, sqrt
# reads a line of input and converts into a list of ints
# 1 1 3 => [1, 1, 3]
def rl():
    return [int(i) for i in raw_input().split()]

# reads n lines of input (if n defined) and returns a list of strings
# where each element is a line in the input
# abc
# abcdef
# => ['abc', 'abcdef']
# if n not defined, read first line to get number of lines
# 2
# abc
# abcdef
# => ['abc', 'abcdef']
def rm(n=None):
    if n is None:
        n = input()
    return [raw_input() for i in range(n)]

# same as rm, except converts each line to a list of ints like rl
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 = int(raw_input())

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