Checkpoint!

Video game rules: save remembers your score, rollback teleports you to the most recent save (using it up). 💾

You start at score 0. You get events — add:10, save, or rollback (only when a save exists). Print the final score.

Example:

Input: events = ["add:10", "save", "add:5", "rollback"] Output: 10

The score reaches 10, save remembers 10, add:5 makes it 15, and rollback teleports back to the saved 10.