There are 45 students in class one of the third grade, and each of them has participated in at least one of the two sports activities of rope skipping and long-distance running. There are 28 people participating in the rope skipping activity and 30 people participating in the long-distance running activity. How many people participated in both activities?
三年级一班有45人，每人至少参加了跳绳、长跑两项体育活动中的一项．参加跳绳活动的有28人，参加长跑活动的有30人，两项活动都参加的有多少人．
<gadget id="calculator">30 - 45</gadget>
<output>-15</output>

<gadget id="calculator">28 + (-15)</gadget>
<output>13</output>

<result>13</result>