A pile of steel pipes has a cross-section that is approximately triangular. It is known that there is 1 steel pipe in the uppermost layer and 4 steel pipes in the bottom layer. There are 4 layers in total (the difference between two adjacent layers is 1 steel pipe). How many steel pipes are there in this pile?
一堆钢管，截面堆成一个近似三角形，已知最上层有1根，最下层有4根，共堆了4层(相邻两层之间相差1根)，这堆钢管共有多少根．
<gadget id="calculator">1 + 4</gadget>
<output>5</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

<result>10</result>