A pile of logs is stacked in a trapezoidal shape. There are 2 logs on the top layer, 15 logs on the bottom layer, and a difference of 1 between adjacent layers. How many logs are there in total in this pile?
一堆原木堆放成梯形状，最上面一层有2根原木，最下面一层有15根，相邻两层之间都是相差1根，这堆原木一共有多少根．
<gadget id="calculator">2 + 15</gadget>
<output>17</output>

<gadget id="calculator">15 - 2</gadget>
<output>13</output>

<gadget id="calculator">13 + 1</gadget>
<output>14</output>

<gadget id="calculator">14 / 2</gadget>
<output>7</output>

<gadget id="calculator">17 * 7</gadget>
<output>119</output>

<result>119</result>