How many digital page numbers are needed for a 40-page book?
一本40页的书，共需要多少个数码编页码？
<gadget id="calculator">40 - 9</gadget>
<output>31</output>

<gadget id="calculator">31 * 2</gadget>
<output>62</output>

<gadget id="calculator">9 + 62</gadget>
<output>71</output>

<result>71</result>