A key can only open one lock. There are currently 8 keys and 8 locks. How many times do you need to try to open them before you can find the corresponding key for each lock?
一把钥匙只能开一把锁．现有8把钥匙和8把锁，最多要试开多少次才能给每把锁找到相应的钥匙．
<gadget id="calculator">7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>28</output>

<result>28</result>