Sub A Add B

Chef has a string of length $N$. The exact contents are irrelevant, only the length matters. Given $2$ parameters $A$ and $B$, Chef does the following repeatedly: - While the string is of length at least $A$, choose a substring of length $A$ from the string and replace it with a substring of length $B$. It is guaranteed that $B < A$, so this process will eventually terminate. Find the final le