We remove the 18 regular customers and their seats from consideration.
Thus, we consider the 23 - 18 remaining customers = 5; and 19 - 18 = 1 seat.
Overbooking means 3 or more of the 5 remaining passengers; each has a 50% chance remaining.
Overbooking = 1 - P(X ≤ 2) = 0.5
Probability of empty seats = P(X ≤ 1) = 0.1875