Quotient of units in the formal power series ring

Question

Let $k[[x,y]]$ be the ring of formal power series in two variables over a field $k$. A unit in $k[[x,y]]$ is of the form $a_0+f$ where $f\in k[[x,y]]$ and $a_0$ is a unit in $k$. I heard that the quotient of two units in $k[[x,y]]$ is again a unit. How do we show this?