A pile of logs is piled up in a trapezoidal shape, with 12 logs in the lower layer and 7 logs in the upper layer. There is one difference between every two adjacent layers. How many logs are there in total in this pile?
一堆圆木，堆成梯形状，下层12根，上层7根，每相邻两层差一根，这堆圆木共有多少根．
<gadget id="calculator">7 + 12</gadget>
<output>19</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">19 * 3</gadget>
<output>57</output>

<result>57</result>