(I posted already that for-loop question from our exam, sorry if this comes out as flooding)

So the question was short and clear, but I can't figure out the answer.

We have four colors (blue and three others) of poker chips in a stack of n chips. In how many ways can the stack be formed so that blue chips are not directly on top of each other?

What I understood by myself:

For example with three chips the options are

1) b o b (o = other, b = blue)

2) o b o

3) o o b

4) b o o

5) o o o

Which gives us 3+9+9+9+27=57 ways.

With two chips the options are

1) b o

2) o b

3) o o

Which gives us 3+3+9 = 15 ways.

But I can't figure out any kind of formula for this.

If someone posted already similar question, a link to that thread would be nice.