An art class required students to draw plants or animals. Among the 36 students in the class, 21 students painted plants and 18 students painted animals. How many people painted both?
一次美术课要求学生画植物或动物，全班36人中，画植物的有21人，画动物的有18人，问两种都画的有多少人？
<gadget id="calculator">18 - 36</gadget>
<output>-18</output>

<gadget id="calculator">21 + (-18)</gadget>
<output>3</output>

<result>3</result>