A pile of logs is piled up in a trapezoidal shape. There are 4 logs on the top layer and 9 logs on the bottom layer. There are 6 layers of logs. How many logs are there in total in this pile?
一堆圆木堆成梯形，最上层有4根，最底层有9根，堆了6层，这堆圆木共有多少根．
<gadget id="calculator">4 + 9</gadget>
<output>13</output>

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

<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<result>39</result>