50 people went on a trip, 31 people agreed to take a minibus, and 28 people agreed to take a van. How many people agree with taking minibuses and vans?
50人去旅游，同意坐中巴车的有31人，同意座面包车的有28人．坐中巴车和面包车都同意的有多少人？
<gadget id="calculator">28 - 50</gadget>
<output>-22</output>

<gadget id="calculator">31 + (-22)</gadget>
<output>9</output>

<result>9</result>