Left inverses, $(m-n) ^n$

Question

Let f be a map from A to B. My book says that there are $(m-n)^n$ left inverses on injective mappings, where $n =|A|$ and $m = |B|$. Can someone explain why this is the case ? $m-n$ is somewhat understandable, since we are looking for all elements of $B$ which are not images of $f$. But why is it then raised to the $n$ ?