In a hotel recruitment exam, there is a question: "Use 20 different keys to open 20 guest room doors. If you don't know which key to open which door, how many times do you have to try at most to match the key and the door lock properly?"
某旅店招工考试，有一道题："用20把不同钥匙开20个客房门，如果不知道哪把钥匙开哪一个门，最多要试开多少次，才能把钥匙与门锁配对妥当．"
<gadget id="calculator">19 + 18 + 17 + 16 + 15 + 14 + 13 + 12 + 11 + 10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>190</output>

<result>190</result>