Ou uses small squares of the same size to build the pagoda, put one on the first floor, and then put two more on each floor than the previous one. It is known that this pagoda has 20 floors in total. So, how many small squares did Ou Ou use to assemble this pagoda?
欧欧用同样大小的小方块拼宝塔，第一层放1个，以后每一层都比前一层多放2个，已知这个宝塔共有20层．那么，欧欧拼成这个宝塔一共用了小方块多少个．
<gadget id="calculator">1 * 20</gadget>
<output>20</output>

<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">2 / 2</gadget>
<output>1</output>

<gadget id="calculator">20 * 19 * 1</gadget>
<output>380</output>

<gadget id="calculator">20 + 380</gadget>
<output>400</output>

<result>400</result>