61 people read books, 42 people read story books, 42 people read technology books, how many people read both kinds of books?
61人看书，42人看了故事书，42人看了科技书，两种书都看的多少人？
<gadget id="calculator">42 - 61</gadget>
<output>-19</output>

<gadget id="calculator">42 + (-19)</gadget>
<output>23</output>

<result>23</result>