How many 0s are needed to arrange the page numbers of a 500-page book?
排一本500页的书的页码，共需要多少个0？
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">1 + 10 + 80</gadget>
<output>91</output>

<result>91</result>