In a pile of wood, there are 4 logs on the top layer and 20 logs on the bottom layer. There is a difference of 2 logs between each adjacent layer. How many logs are there in total in this pile?
一堆木料，最上面一层有4根，最下面一层有20根，每相邻两层之间相差2根，这堆木料共有多少根．
<gadget id="calculator">4 + 20</gadget>
<output>24</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">8 + 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">24 * (9/2)</gadget>
<output>108</output>

<result>108</result>