Non-adjacent subsets

Find the number of subsets of $n$ such that if $i$ does not belong in the subset then either $i+1$ or $i-1$ belongs to the subset. (Hint: use double recursion, use fibonacci series)

Passengers Boarding Airplane, Part Deux

2009 passengers are waiting in a line to board an airplane with 2009 seats. The seats are numbered from 1 to 2009. Each passenger has a boarding pass that has his/her seat number on it. However, these passengers are oblivious to their boarding pass and choose their seats at random, each with a uniform probability from the available empty seats.

What is the probability that everyone's actual seat number and their assigned seat number differ by at most 1? That is, no one sits more than 1 seat away from their assigned seat.