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 sys

def doit(n):
  if n < 1:
    return 0

  if n % 2 == 1:
    x = (n + 1) / 2
    return -x - x * (x - 1)

  x = n / 2

  return x * (x + 1)

def main():
  n = int(sys.stdin.readline().strip())

  print doit(n) + doit(n - 1)


if __name__ == "__main__":
  main()
