A key can only open one lock. Now there are 7 locks and 7 keys, but I don't know which key can open which lock. How many times can I open all the locks?
一把钥匙只能开一把锁，现在有7把锁和7把钥匙，但不知哪把钥匙开哪把锁，最多要试多少次才能把锁全部打开．
<gadget id="calculator">6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>21</output>

<result>21</result>