In a pile of logs, there are 10 logs in the topmost layer and 30 logs in the bottom layer. There is a difference of 1 log in every two adjacent layers. How many logs are there in total in this pile?
一堆圆木，最顶层有10根，最底层有30根，每相邻两层相差1根圆木，这堆圆木一共有多少根．
<gadget id="calculator">10 + 30</gadget>
<output>40</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<gadget id="calculator">21 / 2</gadget>
<output>21/2 = around 10.5</output>

<gadget id="calculator">40 * (21/2)</gadget>
<output>420</output>

<result>420</result>