There is a pile of logs piled up in a trapezoidal shape. There is a difference between every two adjacent layers. There is 1 log on the top layer and 12 logs on the bottom layer. How many logs are there in total in this pile?
有一堆圆木堆成梯形形状，每相邻两层相差一根，最上层有1根，最下层有12根，这堆圆木共有多少根．
<gadget id="calculator">1 + 12</gadget>
<output>13</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">13 * 6</gadget>
<output>78</output>

<result>78</result>