"Story King" in the book corner is divided into one copy for every 5 copies, and one copy for every 4 copies. How many copies of "Story King" are there at least?
图书角的《故事大王》每5本分一份多一本，每4本分一份还是多一本．《故事大王》最少有多少本？
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<result>21</result>