There are 40 students in a class, 17 are interested in mathematics, 13 are interested in music, and 8 are interested in both. How many people are not interested in either?
某班有学生40人，对数学有兴趣的有17人，对音乐有兴趣的有13人，两样都有兴趣的有8人．两样都没有兴趣的有几人？
<gadget id="calculator">40 - 17 - 13</gadget>
<output>10</output>

<gadget id="calculator">10 + 8</gadget>
<output>18</output>

<result>18</result>