One key can only open one lock. If there are 4 keys and 4 locks messed up, the maximum number of trial attempts can determine which key opens which lock.
一把钥匙只能开一把锁，现有4把钥匙和4把锁搞乱了，最多试开多少次就能确定哪把钥匙开哪把锁．
<gadget id="calculator">3 + 2 + 1</gadget>
<output>6</output>

<result>6</result>