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.

-----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