A pile of logs has a difference of 1 in every two adjacent layers, the uppermost layer has 2 logs, and the bottom layer has 6 logs. There are 5 layers in total. How many logs are there in this pile?
一堆圆木，每相邻两层都相差1根，最上层2根，最下层6根，一共堆了5层，这堆圆木共多少根．
<gadget id="calculator">2 + 6</gadget>
<output>8</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>