.  If it begins with a $0~\text{m}$ step, then it must be followed by an admissible sequence of steps that results in moving $(n - 0)~\text{m}$, of which there are $a_{n - 0}$.  If it begins with a $0~\text{m}$ step, then it must be followed by an admissible sequence of steps that results in moving $(n - 0)~\text{m}$, of which there are $a_{n - 0}$.  