A pile of circular plastic tubes, 5 on the top layer and 13 on the bottom layer, with a difference of 1 between every two adjacent layers, how many plastic tubes are there in this pile?
一堆圆形塑料管，顶层有5根，底层有13根，每相邻两层相差1根，这堆塑料管有多少根．
<gadget id="calculator">5 + 13</gadget>
<output>18</output>

<gadget id="calculator">13 - 5</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">18 * (9/2)</gadget>
<output>81</output>

<result>81</result>