Count the number of false positives with respect to the first class
I always find the notion of false positive and negative confusing, especially when it comes to multi-class problems.

A good rule of thumb that I came up with is the following

True positive: "I predicted that you were a certain class, and I was right"

False positive: "I predicted that you were a certain class but I was wrong"

True negative: "I predicted that you weren't a certain class, and I was right"

False negative: "I predicted that you weren't a certain class, but I was wrong"

So in your case, there are 31 true positives with regard to the first class since you predicted 31 times that something was class 1 but you were wrong.