Class 4 and 5 of the Experimental Primary School have five types of books: A, B, C, D, and E. It is stipulated that each student can borrow at most one or two books of different types. Ask at least how many students borrow books to ensure that there are at least two books. Students borrowed the same type of books.
实验小学四5班有A、B、C、D、E五类书，规定每个同学最多可以借一本或两本不同类的书，问至少有多少个同学借书，才能保证至少有两个同学所借的书类型相同．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">4 * (5/2)</gadget>
<output>10</output>

<gadget id="calculator">5 + 10</gadget>
<output>15</output>

<gadget id="calculator">15 + 1</gadget>
<output>16</output>

<result>16</result>