functions - Describing a bijection between ${0,1} times mathbb N$ and $mathbb Z$.

I'm having a bit of trouble formulating a bijection between the sets $\{0,1\} \times \mathbb N$ and $\mathbb Z$. I understand how to find a bijection between $\mathbb N$ and $\mathbb Z$ using a piecewise function that sends even values of $\mathbb N$ to positive integers and odd values of $\mathbb N$ to negative integers, but I'm a bit stuck formulating a function $f(a,n)$ for these two sets. Any help would be greatly appreciated and I apologize for formatting.