Given two strings `s1` and `s2`, find the minimum combined number of character insertions into `s1` and deletions from `s1` needed to turn `s1` into `s2`. The characters that make up the longest common subsequence of `s1` and `s2` never need to be touched — they're already sitting in the right relative order in both strings, so they can act as a fixed skeleton. Everything else has to change: a character of `s1` that isn't part of that shared skeleton has no partner in `s2` and must be deleted, and a character of `s2` that isn't part of it has no partner in `s1` yet and must be inserted. That turns the whole problem into computing one number — the length of the longest common subsequence — and plugging it into a simple formula.