One key can only open one lock. There are 10 keys and 10 locks. How many trials are required to ensure that all the keys match the locks?
一片钥匙只能开一把锁，现有10片钥匙和10把锁，最多要试验多少次能保证全部的钥匙和锁匹配．
<gadget id="calculator">9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>45</output>

<result>45</result>