A Short Proof that the number of $(a,b)$-parking functions of length n $a(a+bn{)}^{n-1}$

We give a very short proof of the fact that the number of $(a,b)$-parking functions of length $n$ equals $a(a+bn{)}^{n-1}$. This was first proved in 2003 by Kung and Yan, via a very long and torturous route, as a corollary of a more general result. This new version contains a reference to previous work kindly communicated by Richard Stanley