In the last semester, 20 people from Class 6 and Class 3 of our school participated in the calligraphy and art interest groups. Among them, 11 people participated in the calligraphy interest group, and 13 people participated in the art interest group. How many people participated in both the calligraphy interest group and the art interest group?
上学期我校六3班有20人参加书法、美术兴趣组．其中参加书法兴趣组的有11人，参加美术兴趣组的有13人，有多少人既参加了书法兴趣组，又参加了美术兴趣组．
<gadget id="calculator">13 - 20</gadget>
<output>-7</output>

<gadget id="calculator">11 + (-7)</gadget>
<output>4</output>

<result>4</result>