In a pile of logs, there are 9 logs in the top layer and 29 logs in the bottom layer, and the difference between every two adjacent layers is 2. How many logs are there in this pile?
一堆圆木，最上层有9根，最下层有29根，而且每相邻两层都相差2根．这堆圆木一共有多少根？
<gadget id="calculator">29 - 9</gadget>
<output>20</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 + 1</gadget>
<output>11</output>

<gadget id="calculator">11 / 2</gadget>
<output>11/2 = around 5.5</output>

<gadget id="calculator">20 * (11/2)</gadget>
<output>110</output>

<result>110</result>