A pile of logs is piled up in a trapezoid shape, with 12 logs in the lower layer and 8 logs in the upper layer, and there is a difference of 1 log in every two adjacent layers. How many logs are there in this pile?
一堆圆木堆成梯形，下层12根，上层8根，每相邻两层差1根，这堆圆木共有多少根．
<gadget id="calculator">12 + 8</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>