Parking problems

Probability Level 4

A parking lot has 10 empty spaces in a row.

6 cars arrive, each of which fills exactly 1 parking spot, chosen at random from among the available spaces. Robbie then arrives in his pick-up truck, which requires 2 empty adjacent spaces to park.

If the probability that Robbie will be able to park is $\frac{a}{b},$ where $a$ and $b$ are coprime positive integers, then what is $a+b?$