There are two interest groups in a certain school. 42 students in the third grade signed up to participate, of which 27 people participated in the calligraphy group, and 24 people participated in the paper-cut group. How many people participated in both groups?
某校开设两个兴趣小组，三年级有42人都报名参加，其中27人参加书法，24人参加剪纸小组，两个小组都参加的有多少人．
<gadget id="calculator">24 - 42</gadget>
<output>-18</output>

<gadget id="calculator">27 + (-18)</gadget>
<output>9</output>

<result>9</result>