Please assist with this probability question for discrete mathematics.

A class with n kids lines up for recess. The order in which the kids line up is random with each ordering being likely. There are two kids in the class named Betty and Mary. The use of the word "or" in the description of the events, should be interpreted as the inclusive or. That is "A or B" means that A is true, B is true, or both A and B are true.

What is the probability that Betty is first in line or Mary is last in line as a function of n? Simplify your final expression as much as possible and include an explanation of how you calculated this probability.