Three classes in the second grade participated in the tree planting activity, and a total of 50 trees were planted. The first class and the second class planted 38 trees, the second class and the third class planted 32 trees, and how many trees were planted in the first class.
二年级三个班参加了植树活动，共植树50棵．一班和二班植树38棵，二班和三班植树32棵，一班植树多少棵．
<gadget id="calculator">32 - 50</gadget>
<output>-18</output>

<gadget id="calculator">38 + (-18)</gadget>
<output>20</output>

<gadget id="calculator">38 - 20</gadget>
<output>18</output>

<result>18</result>