Bus No. 4 got off 15 people and boarded 17 people at station A, and got off 13 people at station B, and there were 21 people on the bus. How many people are there on the No. 4 bus?
4路公交到A站下了15人上了17人，到了B站又下了13人，车上还有21人．4路公交车上原有多少人．
<gadget id="calculator">13 - 17</gadget>
<output>-4</output>

<gadget id="calculator">21 + (-4) + 15</gadget>
<output>32</output>

<result>32</result>