There are 23 people in the class who can swim, 18 people who can roller skate, 9 people who can do both, and 13 people who can't do both. How many people are there in the class?
班上有23人会游泳，18人会轮滑，这两样都会的有9人，都不会的有13人，全班共有多少人．
<gadget id="calculator">18 - 9</gadget>
<output>9</output>

<gadget id="calculator">23 + 9 + 13</gadget>
<output>45</output>

<result>45</result>