How do I use the KUNAMI CODE?

I am trying to add a secret menu to my game that is activated by using the Kunami Code (UPUPDOWNDOWNLEFTRIGHTLEFTRIGHTBAEnter) I know how to register multiple keys being held, but how do I detect if certain keys are hit one after another in a certain amount of time. I was thinking possible to have a countdown variable and if all the keys are hit before it hits 0 it would open the menu, but I don't have a clue how to make it detect if they are hit in the right order. Thanks for the help!

Here is a simple way of doing this:

I didn't even think of using a string to store typed characters. Brilliant. Thanks a ton dop2000