One key can only open one lock, and now there are 10 keys and 10 locks messed up, so how many times can you try to open them at most to ensure that all the keys and locks are re-matched.
一把钥匙只能开一把锁，现在有10把钥匙和10把锁弄乱了，那么最多试开多少次就可以确保把钥匙和锁全部重新配好．
<gadget id="calculator">9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>45</output>

<result>45</result>