The lumberjacks piled the felled logs into a pile. The cross-section is a trapezoid. It is known that there are 9 logs on the top layer and 31 logs on the bottom layer. There are 23 layers in total. Come and do the calculations for this pile of logs. How many roots are there in total.
伐木工人把伐下的圆木，堆成一堆，横截面是一个梯形，已知最上层有9根，最下层有31根，共堆了23层，你来帮忙算一算，这堆圆木共有多少根．
<gadget id="calculator">9 + 31</gadget>
<output>40</output>

<gadget id="calculator">23 / 2</gadget>
<output>23/2 = around 11.5</output>

<gadget id="calculator">40 * (23/2)</gadget>
<output>460</output>

<result>460</result>