Sequence: 1, 3, 2, 4, 3, 5, ... what is the 20th number.
数列：1，3，2，4，3，5，…的第20个数是多少．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 * 1</gadget>
<output>9</output>

<gadget id="calculator">3 + 9</gadget>
<output>12</output>

<result>12</result>