Alternating Strings

Chef likes to spend his summer vacation playing with strings. Chef likes alternating patterns, but he only likes the characters $\texttt{a}$ and $\texttt{b}$, so he does the following: Chef starts with an empty string $S$. Then, - First, he appends $\texttt{a}$ once to $S$ - Next, he appends $\texttt{b}$ twice to $S$ - Next, he appends $\texttt{a}$ thrice to $S$ - Next, he appends $\texttt{b}$ f