How many digits "0" are needed to arrange the page numbers of a 400-page book?
排一本400页的书的页码，共需要多少个数码"0"．
<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">1 + 10 + 60</gadget>
<output>71</output>

<result>71</result>