A key can only open one lock. There are 7 keys and 7 locks, but it is unknown which key can open which lock. How many times can you try to match all the keys and locks?
一把钥匙只能开一把锁，现有7把钥匙和7把锁，但不知道哪把钥匙开哪把锁．最多要试多少次才能配好全部的钥匙和锁．
<gadget id="calculator">6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>21</output>

<result>21</result>