Random variables and expected value

Hello, I'm really stuck on this problem:

*There are k types of coupons. Independently of the types of previously collected coupons, each new coupon collected is of type i with probability pi. If n coupons are collected, find the expected number of distinct types that appear in this set. (That is, find the expected number of types of coupons that appear at least once in the set of n coupons.) *

I'm not sure how to set up this problem. I know that the random variable should be X=#of distinct types of stamps, but I am not sure where to go from there.

Thanks for the help in advance!