I tried working this out, but I think the original was correct. The cases where the answer is negative correspond to a 'runaway scenario' where every change is

I tried working this out, but I think the original was correct. The cases where the answer is negative correspond to a 'runaway scenario' where every change is expected to cause more than 1 extra change. So the answer is 'nonsensical' (because that is indeed where the formula for geometric series no longer works) but the true answer is infinity.