There is a pile of logs piled up to form a trapezoidal cross-section. There are 2 logs on the top layer and 8 logs on the bottom layer. There is a difference between every two adjacent layers. How many logs are there in this pile?
有一堆圆木堆成横截面是梯形的木堆，最上层有2根，最下层有8根，每相邻两层相差一根，这堆圆木共有多少根？
<gadget id="calculator">2 + 8</gadget>
<output>10</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">10 * (7/2)</gadget>
<output>35</output>

<result>35</result>