There are 50 students in class 3 and 6, all of whom participated in interest groups, including 28 people in the art group and 33 people in the music group. How many people participated in both interest groups?
三6班有50人，都参加了兴趣小组，其中参加美术组的有28人，参加音乐组的有33人．两种兴趣小组都参加的有多少人？
<gadget id="calculator">33 - 50</gadget>
<output>-17</output>

<gadget id="calculator">28 + (-17)</gadget>
<output>11</output>

<result>11</result>