HuaXin bookstore is not a famous bookstore. However, it still attracts many people to go there every day. Now, there are several kinds of books in HuaXin bookstore. Each kind of books has an amount of ai (1 <= ai <= 20). People comes to HuaXin bookstore to find the book which he/she wants to read. But after statistics, the boss found that some customers may leave without having a look of the book if there is no book available.In addition, the customers who enters with no book to read will only hang in the bookstore without looking any book before he leave. Your task is to write a program to tell the boss how many customers left without having a look of the books.