The school has set up two interest groups. All 45 students from Class 3 and Class 1 signed up. Among them, 28 people participated in the calligraphy and painting group, and 26 people participated in the chess group. How many people participated in both groups?
学校开设了两个兴趣小组，三1班45人全部报名参加，其中有28人参加了书画小组，有26人参加了棋艺小组，两个小组都参加的有多少人．
<gadget id="calculator">26 - 45</gadget>
<output>-19</output>

<gadget id="calculator">28 + (-19)</gadget>
<output>9</output>

<result>9</result>