The school asked the students to determine the location of the autumn outing. Among the 45 students in the class, 27 people like Beihai Park, and 30 people like the Summer Palace. How many people like both places?
学校让同学们自己确定秋游地点，全班45人中，有27人喜欢北海公园，有30人喜欢颐和园，有多少人两个地点都喜欢？
<gadget id="calculator">30 - 45</gadget>
<output>-15</output>

<gadget id="calculator">27 + (-15)</gadget>
<output>12</output>

<result>12</result>