Let's say the knight is on a1, and number of moves is 2. It seems that the result includes b3, but it's impossible to get from a1 to b3 in exactly 2 moves. So, I suppose the intended semantics is "at most n moves".

But then, when n=1, why is a1 not in the set?? You can get from a1 to a1 in at most 1 step: namely, in 0 steps. Zero is a number. I really think the task is inconsistent, requiring the understanding of exactly one step when n=1, and at most two steps when n=2. :-/ Please fix.