There are 28 students from class 31 who participated in the art interest group, 35 students who participated in the music interest group, 15 students who participated in both items, and 4 students who did not participate in both items. How many students are there in class 31?
三1班同学参加美术兴趣小组的有28人，参加音乐兴趣小组的有35人，两项都参加的有15人，两项都没有参加的有4人，三1班共有学生多少人．
<gadget id="calculator">35 - 15</gadget>
<output>20</output>

<gadget id="calculator">28 + 20 + 4</gadget>
<output>52</output>

<result>52</result>