There are three kinds of books: science and technology books, literature and art books, and story books. Each student can borrow two books at will, and at least how many students can borrow books to ensure that there must be 4 people who borrow the same type of books during the term.
有三种图书：科技书、文艺书、故事书．每位同学可任借两本，至少多少位同学借书，才能保证期中必有4人借的书类型相同．
<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">18 + 1</gadget>
<output>19</output>

<result>19</result>