11 students go to the teacher's home to borrow books. The teacher has four types of books, A, B, C, and D in the study room. Each student can borrow at most two books of different types, and at least one. How many students must borrow the same type of books.
11名学生到老师家借书，老师是书房中有A、B、C、D四类书，每名学生最多可借两本不同类的书，最少借一本．必有多少个学生所借的书的类型相同．
<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>