The hotel now has 9 single rooms, and 10 travelers may stay. 9 of these 10 passengers check in at the same time. The administrator wants to assign some keys to each of them in advance, so that no matter which 9 people check in, they can always stay in these 9 rooms, and there is no need to ask others to borrow keys. Excuse me: at least How many keys are needed?
旅店现在有9个单人间，10名旅客可能入住．这10名旅客每次有9个人同时入住，管理员想事先给每个人配一些钥匙，使得无论是哪9个人入住，总能正好入住这9个房间，而且不用找别人借钥匙，请问：最少需要多少把钥匙？
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>