A bus runs from the starting station to the terminal station, and there are 9 stops on the way. If the bus leaves from the starting station, except for the terminal station, among the passengers getting on the bus at each station, the number of passengers from this station to the following Exactly one passenger gets off at each stop. How many seats must the bus have in order for each passenger to have a seat?
某公共汽车从起点站开往终点站，中途共有9个停靠点，如果这辆公共汽车从起点站开出，除终点站外，每一站上车的乘客中，从这一站到以后的每一站正好各有一个乘客下车．为了使每位乘客都有座位，这辆公共汽车至少有座位多少个？
<gadget id="calculator">10 + 9 + 8 + 7 + 6</gadget>
<output>40</output>

<gadget id="calculator">0 + 1 + 2 + 3 + 4</gadget>
<output>10</output>

<gadget id="calculator">40 - 10</gadget>
<output>30</output>

<result>30</result>