theater has 20 rows of seats, the first row has 30 seats, and each subsequent row has 2 more seats than the previous row. How many seats are there in total in this theater?
一个剧场设置了20排座位，第一排30个座位，以后每一排都比前一排多2个座位，这个剧场一共有多少个座位．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

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

<gadget id="calculator">30 + 38</gadget>
<output>68</output>

<gadget id="calculator">30 + 68</gadget>
<output>98</output>

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

<gadget id="calculator">98 * 10</gadget>
<output>980</output>

<result>980</result>