A pile of logs is piled up to form a trapezoidal cross-section. There are 2 logs in the upper layer and 6 logs in the lower layer. The difference between every two adjacent layers is 1 log. How many logs are there in total in this pile?
一堆圆木堆成一个横截面是梯形的形状，上层2根，下层6根，每相邻两层相差都是1根，这堆圆木共有多少根．
<gadget id="calculator">2 + 6</gadget>
<output>8</output>

<gadget id="calculator">6 - 2</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">8 * (5/2)</gadget>
<output>20</output>

<result>20</result>