28 people from Class 1 participated in the Chinese and mathematics competitions. There are 15 people who participated in the Chinese competition, 18 people participated in the mathematics competition, and how many people participated in both the language and mathematics competitions.
六1班有28人参加了语文和数学竞赛．参加语文竞赛的有15人，参加数学竞赛的有18人，语数竞赛都参加的有多少人．
<gadget id="calculator">18 - 28</gadget>
<output>-10</output>

<gadget id="calculator">15 + (-10)</gadget>
<output>5</output>

<result>5</result>