question on functions

The function is defined as follows;
$f=\mathbb{Z}^{+}\times\{0,1\}\rightarrow\mathbb{Z}$ by

$$f(n,m)=\begin{cases}n-1 & \text{if }m=0\\ -n & \text{if }m=1\end{cases}.$$
I have to show this function is a bijection and then to find the inverse of f.

How do I start this?

Thank you.