A pile of steel pipes is stacked in a trapezoidal shape, with 8 on the top layer and 12 on the bottom layer, with a difference of one between each adjacent layer. How many steel pipes are there in total?
一堆钢管堆放成梯形，顶层8根，底层12根，每相邻两层相差一根，一共有钢管多少根．
<gadget id="calculator">8 + 12</gadget>
<output>20</output>

<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">20 * (5/2)</gadget>
<output>50</output>

<result>50</result>