The school library has three kinds of books: popular science books, story books and comic books. Each student borrows two books from it. Then at least how many students need to borrow books to ensure that there must be two of them who have borrowed books of the same category.
学校图书馆有科普读物、故事书、连环画三种图书．每个学生从中借阅两本．那么至少要多少个学生借阅才能保证其中一定有2个人所借阅的图书属于同一种类．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>