In a square playground, 18 trees are planted on each side (1 tree is planted at each of the 4 corners), how many trees are planted in total?
一个正方形操场，每边栽18棵(4个角各栽1棵)，共栽多少棵．
<gadget id="calculator">18 * 4</gadget>
<output>72</output>

<gadget id="calculator">72 - 4</gadget>
<output>68</output>

<result>68</result>