A book has 400 pages, number the pages 1, 2, 3, 4, ..., and ask how many times the number 2 appears in the page number.
一本书有400页，编上页码1、2、3、4、…，问数字2在页码中出现多少次．
<gadget id="calculator">10 + 10</gadget>
<output>20</output>

<gadget id="calculator">100 + 20</gadget>
<output>120</output>

<gadget id="calculator">2 * 20</gadget>
<output>40</output>

<gadget id="calculator">20 + 120 + 40</gadget>
<output>180</output>

<result>180</result>