@xkcd I was wondering what numbers would be needed to run out of both numbers at the same time, so I started at 1 2 and 1 3 and worked backward. Interestingly, there are steps where there could be more than predecessor. So it isn't just what values would they need to start with, but also how many pairs of values would work (within a given bounds; say numbers where both quantities are two digits but don't both remain two digits after the first iteration.)