Showing $\int_{a}^{b} \left\lfloor x \right\rfloor dx + \int_{a}^{b} \left\lfloor -x \right\rfloor dx=a-b$ I want to show

$$\int_{a}^{b}  \left\lfloor x \right\rfloor dx  +  \int_{a}^{b} \left\lfloor -x \right\rfloor dx=a-b$$

I know that
\begin{equation}
\left\lfloor -x \right\rfloor = \begin{cases} -\left\lfloor x \right\rfloor & \text{if } x \in \mathbb{Z} \\ -\left\lfloor x \right\rfloor-1 & \text{if } x \notin \mathbb{Z}. \end{cases} 
\end{equation}
In this case do I use $-\left\lfloor x \right\rfloor$ or $-\left\lfloor x \right\rfloor-1$? I think I am confused about some definitions, one of the solutions said $\left\lfloor x \right\rfloor$ is constant on the open subintervals of the partition 
$$P=\left(a, \left\lfloor a \right\rfloor+1 \cdots \left\lfloor a \right\rfloor + \left\lfloor b-a \right\rfloor, b\right)$$ 
and since there are no integers in the open subintervals of P, then we would use $-\left\lfloor x \right\rfloor - 1$.. I don't think I quite understand this point here. I know I can solve it and say 
\begin{align*}
\int_{a}^{b}  \left\lfloor x \right\rfloor dx  +  \int_{a}^{b} \left\lfloor -x \right\rfloor dx= \int_{a}^{b}  \left\lfloor x \right\rfloor dx  +  \int_{a}^{b} -\left\lfloor x \right\rfloor -1 \; dx= a-b
\end{align*}
but I don't understand why. 

What happens at the integers does not matter since they have content 0.  Suppose 
$$n < x < n + 1.$$
then $\lfloor x \rfloor = n$, and $-n > x > -(n+1)$, so $\lfloor -x \rfloor = -(n+1)$.  Adding gives $\lfloor x\rfloor + \lfloor -x \rfloor = -1$.  
