There are four types of books in the school reading room: A, B, C, and D. It is stipulated that each student can borrow at most 3 books (two books and three books must be of different types). The borrowed books are the same?
学校阅览室有甲、乙、丙、丁四类读物，规定每个同学最多可以借3本(两本、三本要不同类)，至少有多少个同学借书，才能保证至少有两人所借的书相同？
<gadget id="calculator">14 + 1</gadget>
<output>15</output>

<result>15</result>