# Billiard (noch nicht übersetzt)

Problem 786

The following diagram shows a billiard table of a special quadrilateral shape. The four angles $A, B, C, D$ are $120^\circ, 90^\circ, 60^\circ, 90^\circ$ respectively, and the lengths $AB$ and $AD$ are equal.

The diagram on the left shows the trace of an infinitesimally small billiard ball, departing from point $A$, bouncing twice on the edges of the table, and finally returning back to point $A$. The diagram on the right shows another such trace, but this time bouncing eight times:

The table has no friction and all bounces are perfect elastic collisions.
Note that no bounce should happen on any of the corners, as the behaviour would be unpredictable.

Let $B(N)$ be the number of possible traces of the ball, departing from point $A$, bouncing at most $N$ times on the edges and returning back to point $A$.

For example, $B(10) = 6$, $B(100) = 478$, $B(1000) = 45790$.

Find $B(10^9)$.