A movie theater has 20 rows of seats. It is known that the first row has 10 seats, and the front row has 1 seat less than the back row. How many seats does this movie theater have?
一个电影院共有20排座位，已知第一排有10个座位，前排比后排少1个座位，这个电影院一共有多少个座位．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">19 * 1</gadget>
<output>19</output>

<gadget id="calculator">10 + 19</gadget>
<output>29</output>

<gadget id="calculator">10 + 29</gadget>
<output>39</output>

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

<gadget id="calculator">39 * 10</gadget>
<output>390</output>

<result>390</result>