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.

Note

$*f*(4) =  - 1 + 2 - 3 + 4 = 2$

$*f*(5) =  - 1 + 2 - 3 + 4 - 5 =  - 3$