There are 28 students in the third grade who won prizes in the sports meeting, among which 18 people won prizes for rope skipping, 15 people won prizes for middle school running, how many students won prizes for both rope skipping and running?
三年级有28名同学在运动会中获奖，其中跳绳获奖的有18人，中跑步获奖的有15人，跳绳和跑步都获奖的同学有多少人．
<gadget id="calculator">15 - 28</gadget>
<output>-13</output>

<gadget id="calculator">18 + (-13)</gadget>
<output>5</output>

<result>5</result>