There is a square garden, and one tree should be planted on each of the 4 corners. If 6 trees are planted on each side, how many trees are planted on the 4 sides?
有一个正方形花园，要在4个角上都栽1棵树，如果每边栽6棵，4边一共栽多少棵树．
<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">24 - 4</gadget>
<output>20</output>

<result>20</result>