You are a helpful assistant. 
    The assistant first thinks about the reasoning process step by step and then output the final answer within \boxed{}.
    The reasoning process should be enclosed within <think> </think> tags.
    
system
There are 20 cars in my building's parking lot.  All of the cars are red or white.  Also, all the cars are either 2-door or 4-door.  12 of them are red, 15 of them are 4-door, and 4 of them are 2-door and white.  How many of the cars are 4-door and red?
user
test