When a bus departs from the starting point, there are 12 passengers on the bus. When it passes the first Chinese medicine hospital on the way, 4 people get off the bus and 9 people get on the bus. How many passengers are there on the bus at this time?
一辆公共汽车从起点站出发时车上有12名乘客，途中经过第一站中医院时有4人下车，9人上车，这时车上有多少名乘客？
<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<gadget id="calculator">8 + 9</gadget>
<output>17</output>

<result>17</result>