Do you know if the maths rule of negatives and positives applies? Eg. negative and negative = plus. Sounds really stupid but if unsure, no worries.
That's actually a good question, and the answer is a little bit complicated because opticians use two different forms of written refraction.
If you're long-sighted, and your prescription is of the form +A.AA +B.BB, then the refractive error in the worst median is A.AA + B.BB.
If your prescription is of the form +C.CC -D.DD, then the refractive error in the worst meridian is C.CC.