Getting on a plane

2017 passengers are boarding a plane that seats 2017 people. Each passenger has an assigned seat. However, the first three passengers do not sit in their assigned seats, and instead sit in a random empty seat. After that, each subsequent passenger sits on their assigned seat, unless it is already occupied, in which case they also sit on a random empty seat. What is the probability that the last passenger to board the plane sits on his assigned seat?

If you think the answer is $\frac{a}{b}$ , where $a$ and $b$ are coprime positive integers, input your answer as $a+b$ .