The students of Class 6 and 1 went to Xinhua Bookstore to buy "Journey to the West", "Romance of the Three Kingdoms", and "Animal World". The situation of buying books was as follows: some bought one book, some bought two books, and some bought three books. At least how many students must buy books to ensure that two students must buy the same book? (Each student only buys one book of each type)
六1班学生到新华书店去买《西游记》、《三国演义》、《动物世界》，买书的情况是：有买一本书的，有买两本书的，也有买三本书的，至少要有多少位同学买书，才能保证一定有两位同学购买的书相同？(每位同学每种书只买一本)
<gadget id="calculator">3 + 3 + 1</gadget>
<output>7</output>

<gadget id="calculator">7 + 1</gadget>
<output>8</output>

<result>8</result>