A pair of cylindrical steel pipes are neatly stacked together. There are 10 steel pipes in the top layer and 30 steel pipes in the bottom layer. There is a difference of 1 steel pipe in every two adjacent layers. How many steel pipes are there in this pile?
一对圆柱形钢管整齐地堆放在一起，它的最上层有10根，最下层有30根，每相邻两层相差1根，这堆钢管一共有多少根？
<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>