It’s twice as long. You had to climb three flights to get from the first floor to the fourth; now you must climb six flights to get from the first to the seventh.

That was the solution intended by Russian mathematician Boris Kordemsky, who invented the puzzle. It’s not valid in Britain, interestingly, which counts the ground-level story as the “ground floor,” the one above that as the first, etc. Evidently Russians and Americans have more in common than we thought.