A theater has 20 rows of seats, the last row has 2 more seats than the first row, and the last row has 50 seats. How many seats are there in total in this theater?
某剧院有20排座位，后一排比一排多2个座位，最后一排有50个座位，这剧院共有多少个座位．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

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

<gadget id="calculator">50 - 38</gadget>
<output>12</output>

<gadget id="calculator">50 + 12</gadget>
<output>62</output>

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

<gadget id="calculator">62 * 10</gadget>
<output>620</output>

<result>620</result>