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
#
# Author: eloyhz
# Date: Sep/08/2020
#

from math import ceil


def s(n):
    return n * (n + 1) // 2


if __name__ == '__main__':
    n = int(input())
    even = 2 * s(n // 2)
    odd = ceil(n / 2) + 2 * s(n // 2 + (0 if n % 2 else -1))
    print(even - odd)

