One key can only open one lock. Now there are 10 keys and 10 locks. How many trials can be done to match all the keys and locks?
一把钥匙只能开一把琐，现在有10把钥匙和10把锁，最多试验多少次才能配好全部的钥匙和锁．
<gadget id="calculator">9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>45</output>

<result>45</result>