One key can only open one lock. There are 8 keys and 8 locks. How many trials are needed to match all the locks?
一片钥匙只能开一把锁，现有8片钥匙和8把锁，最多要试验多少次能使全部的锁匹配．
<gadget id="calculator">7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>28</output>

<result>28</result>