One up, Two down seems like a very simple puzzle game. You have to create numbers from 1 to 999 using the three digits, just like in a combination lock.

First you increase one digit by one, then you decrease a digit by two. Then you increase a digit again, then decrease a digit by two and so on. What makes it tricky is that you can't modify one digit two times in a row. This means you really have to work hard to get the next number on your way to 999!