45 students go on an outing, 26 students wear umbrellas, 28 students wear hats, how many students wear both umbrellas and hats?
45名同学去郊游，带伞的有26名，带帽子的有28名，既带伞又带帽子的同学有多少名？
<gadget id="calculator">28 - 45</gadget>
<output>-17</output>

<gadget id="calculator">26 + (-17)</gadget>
<output>9</output>

<result>9</result>