I am a first-year physics major (currently in calc 2) and was wondering if there are any good introductory/low-level books on combinatorics through which I could educate myself over the summer. My college is offering a topics course on it next semester, but I was told by the professor that it has high prerequisites and I probably wouldn't understand it at the level that it will be taught at. Because my college is a small institution, it may not be taught again before I graduate, which is why I'm looking into studying it myself. Any tips or suggestions? Also, will it be difficult to learn the majority of combinatorics topics with only a calc 2 background?