A string arranged according to the law: 2, 5, 9, 14, ..., what is the 19th number in this string.
按规律排列的一串：2，5，9，14，…，这串数的第19个数是多少．
<gadget id="calculator">1 + 20</gadget>
<output>21</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">21 * 10</gadget>
<output>210</output>

<gadget id="calculator">210 - 1</gadget>
<output>209</output>

<result>209</result>