One key can only open one lock. There are 4 keys and 4 locks, but I don’t know which key can open which lock. Now all 4 locks are locked. How many times can I open all the locks?
一把钥匙只能开一把锁，现有4把钥匙4把锁，但不知哪把钥匙开哪把锁，现在4把锁全锁着，最多试多少次可把锁全部打开？
<gadget id="calculator">4 + 3 + 2 + 1</gadget>
<output>10</output>

<result>10</result>