30 students signed up for the art group. Among them, 26 people participated in the art group and 17 people participated in the calligraphy group. How many people participated in both groups?
30名学生报名参加美术小组．其中有26人参加了美术组，17人参加了书法组．问两个组都参加的有多少人？
<gadget id="calculator">17 - 30</gadget>
<output>-13</output>

<gadget id="calculator">26 + (-13)</gadget>
<output>13</output>

<result>13</result>