There are a pile of steel pipes of the same type stacked into a trapezoid, with 9 on the top layer and 18 on the bottom layer, with a difference of one in each layer. How many steel pipes are there in total?
有一堆同一型号的钢管堆成梯形，顶层有9根，底层有18根，每层相差一根．这堆钢管一共有多少根？
<gadget id="calculator">9 + 18</gadget>
<output>27</output>

<gadget id="calculator">18 - 9</gadget>
<output>9</output>

<gadget id="calculator">9 + 1</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">27 * 5</gadget>
<output>135</output>

<result>135</result>