Good game! I enjoyed the first puzzle most, mainly because the second was a pain to write and I didn't think to allow backtracking in the second. I'm stuck, tho

Good game! I enjoyed the first puzzle most, mainly because the second was a pain to write and I didn't think to allow backtracking in the second. I'm stuck, though, on the bonus puzzle; Is there a hint I'm missing? Brute-force doesn't seem a sensible answer.