In a pile of steel pipes, there are 3 steel pipes in the uppermost layer, 15 steel pipes in the lowermost layer, and the difference between two adjacent layers is 1 steel pipe. How many steel pipes are there in total in this pile?
一堆钢管，最上层有3根，最下层有15根，相邻两层相差1根，这堆钢管一共有多少根．
<gadget id="calculator">3 + 15</gadget>
<output>18</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">12 + 1</gadget>
<output>13</output>

<gadget id="calculator">13 / 2</gadget>
<output>13/2 = around 6.5</output>

<gadget id="calculator">18 * (13/2)</gadget>
<output>117</output>

<result>117</result>