Plant trees around a square pond, 18 trees are planted on each side, and one tree is planted on each of the four vertices. How many trees are planted around this pond?
在一块正方形池塘的四周种树，每边都种18棵，并且四个顶点都种有一棵．求这个池塘四周一共种树多少棵？
<gadget id="calculator">18 * 4</gadget>
<output>72</output>

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

<result>68</result>