A pile of logs, piled up into a pile with a trapezoidal cross-section, each layer has one more log than the upper layer. The bottom layer is 12, and the top layer is 8. How many logs are there in this pile?
一堆圆木，堆成截面是梯形的一堆，每一层都比上层多一根．最下层12根，最上层8根．这堆圆木共有多少根．
<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>