A pile of steel pipes arranged in a trapezoidal shape has 7 steel pipes in the bottom layer and 2 steel pipes in the uppermost layer. The difference between each two adjacent layers is 1 steel pipe. How many steel pipes are there in this pile?
一堆成梯形摆放的钢管最下层有7根，最上层有2根，每相邻的两层都相差1根，这堆钢管共有多少根．
<gadget id="calculator">2 + 7</gadget>
<output>9</output>

<gadget id="calculator">7 - 2</gadget>
<output>5</output>

<gadget id="calculator">5 + 1</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<result>27</result>