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 sys import stdin, maxsize
from math import ceil

lines, line_index = stdin.readlines(), -1

def get_line():
  global lines, line_index

  line_index += 1

  return lines[line_index]

def even_sum(n):
  if n % 2 != 0:
    n = n - 1

  N = (2 + n) // 2 - 1

  return N * (N + 1)

def odd_sum(n):
  t = ceil(n / 2.0)
  return int(t ** 2)

def main():
  n = int(get_line())

  s1 = even_sum(n)
  s2 = odd_sum(n)

  print(s1 - s2)

main()