A pile of water pipes, from the bottom to the upper layer, there is one less than the first layer, 13 are placed on the lower layer, and 5 are placed on the upper layer, making a total of 9 layers. How many water pipes are there in this pile?
一堆水管，从下到上一层比一层少1根堆放，下层放了13根，上层放了5根，一共放了9层，这堆水管共有多少根．
<gadget id="calculator">5 + 13</gadget>
<output>18</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>