<s>[INST] The price of fetching one book from the crime section in a library is $0.50, two books is $0.80, three books is $0.90, four books from the same section is still $0.90, and the price stays constant at $0.90 for any more than four books from the crime section. Find the optimal integer number of books that a reader should fetch such that the average price per book is minimized.[/INST]