There are 20 numbers, the first number is 9, and each subsequent number is 2 greater than the previous number, what is the 20th number
有20个数，第一个数是9，以后每一个数都比前一个数大2，第20个数是多少
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">19 * 2</gadget>
<output>38</output>

<gadget id="calculator">9 + 38</gadget>
<output>47</output>

<result>47</result>