I know the basics of logic, sets, relations and the like, so studying intros to abstract algebra and real analysis is not that hard. That said, I have a deficiency when it comes to elementary number theory and combinatorics topics. Things like divisibility, modular arithmetic, congruences, prime numbers and such like regularly pop up as examples to illustrate certain topics in analysis.

What are some very concise books or, maybe, online resources for learning these topics from number theory and combinatorics? Since I need to know them to understand what's going on in other subjects I probably need the bare minimum.