Plant trees around a rectangular playground. 12 trees should be planted on the long side, 8 trees should be planted on the wide side, and one tree should be planted at each vertex. How many trees should be planted in total?
在一个长方形操场的四周种树，长边上要种12棵树，宽边上要种8棵树，每个顶点都种一棵，总共要种多少棵树．
<gadget id="calculator">12 + 8</gadget>
<output>20</output>

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">40 - 4</gadget>
<output>36</output>

<result>36</result>