There is a square playground with 6 trees planted on each side and 1 tree at each of the four corners. How many trees are planted in total?
有一个正方形操场，每边都栽6棵树，四个角各栽1棵，一共栽了多少棵树？
<gadget id="calculator">6 * 4</gadget>
<output>24</output>

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

<result>20</result>