- I have two symbols, one is "long" 1 bit and one is "long" 2 bits. That is X and YY

- The first has probability 2/3 and the second 1/3

- I have a string of these symbols, for example "X YY X X YY YY X X..."

My question: Given a length L, how do I find how many different strings exist of such length? In other words how many different strings I can build such the the length of all its symbols is exactly L

Thanks for the help.