Checkmate (aka the urinal problem)
Oasis, 5 bytes

Code

cd+2V


Extended version

cd+211


Explanation

1 = a(0)
1 = a(1)
2 = a(2)

a(n) = cd+
       c      # Calculate a(n - 2)
        d     # Calculate a(n - 3)
         +    # Add them up


Try it online!