25 people from class 51 participated in the school's calligraphy group and dance group, of which 15 people participated in the calligraphy group, and 18 people participated in the dance group. How many people participated in both groups?
五1班有25人参加了学校的书法小组和舞蹈小组，其中15人参加了书法小组，有18人参加了舞蹈小组，有多少人这两个小组都参加．
<gadget id="calculator">18 - 25</gadget>
<output>-7</output>

<gadget id="calculator">15 + (-7)</gadget>
<output>8</output>

<result>8</result>