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 math import floor
"""
For a positive integer n let's define a function f:

f(n) =  - 1 + 2 - 3 + .. + ( - 1)^n*n

Your task is to calculate f(n) for a given integer n.

Input
The single line contains the positive integer n (1 ≤ n ≤ 10^15).

Output
Print f(n) in a single line.
"""
def special_sum(n):
    if n%2 == 0:
        return 1 * int(n/2)
    else:
        return 1 * int((n-1)/2) - n

number = int(input())
print(special_sum(number))