It depends a bit on the padding itself. E.g. if you know that the padding will be applied to all spatial sides with a constant value, you could just grab the very first value, and remove it via indexing. However, since you cannot access a and b, I assume you know the padding size?
If so, you could index a using this size, but again you are apparently not allowed to use a.
Could you clarify this restriction a bit, since you won’t be able to create c without using a or b at all?