Solution
= Solution
The two possible colour orders are $GGRR$ and $RRGG$. Thus the answer is
$$
\boxed{2}.
$$
They correspond to the positive and negative four-step excursions with <first return time> four.
= Solution
The two possible colour orders are $GGRR$ and $RRGG$. Thus the answer is
$$
\boxed{2}.
$$
They correspond to the positive and negative four-step excursions with <first return time> four.