One key can open one lock. Now there are 5 keys and 5 locks. I don’t know which key belongs to which lock. If all the keys match the lock, what is the maximum number of attempts to open it?
一把钥匙能开一把锁，现在又5把钥匙5把锁，不知道那个钥匙是那把锁的，若全部的钥匙和锁匹配，试开的最多次数是多少．
<gadget id="calculator">4 + 3 + 2 + 1</gadget>
<output>10</output>

<result>10</result>