-- Category --
Reverse

-- Author --
Ajax

-- Level --
400

-- Instructions --
Find a valid key.
Because there is an infinite number of answer, in order to submit you need to:
* Use only characters '0', '1', '2', '3' and a last one of your choice
* Submit the smaller key as possible
* Prefer '3' to '2', '2' to '1' and '1' to '0'