Storing n books on a bookshelf, where the length of each shelf is L. The order of the books is fixed and cannot be rearranged. Therefore, we can speak of a book b_i, where 1 i n, that has a thickness t_i and height h_i. Assume there is enough space to store all the books regardless of the book placement plan.

Define the cost of a book placement plan to be the number of shelves that it uses.