I'm in the process of teaching myself Objective-C via Stephen Kochan's "Programing In Objective-C 2.0" It's a great book and currently reading it for the second time, first time I got the gist of it and now it's really starting to sink in the second time. 

If you have a moderate (about) knowledge of Objective-C, how difficult would it be to learn C? I realize there is seemingly endless debate on which language to learn first. I decided to go with Objective-C because I was interested in Cocoa Mac Apps/iPhone apps.

Side Note: For those familiar with the Chipmunk Physics engine... The reason I may start pursuing C eventually is that it uses C. How much C would I need to know to adequately use it. I was going to use it along with Cocos2d which uses Objective-C