A key can only open one lock. Now there are 10 keys and 10 locks, but I don't know which key will open which lock. How many times will it take to successfully pair all the keys and locks?
一把钥匙只能开一把锁．现在有10把钥匙10把锁，但不知哪把钥匙开哪把锁．最多要试多少次才能将所有的钥匙和锁成功配对．
<gadget id="calculator">9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>45</output>

<result>45</result>