A turtle challenges the Greek warrior Achilles to a race outside the gates of Troy. Achilles accepts. The turtle and Achilles start from the gates of Troy and race in a straight line away from the city. They keep a constant speed throughout the race. During the race, Achilles is f meters from the gates, while the turtle is only g meters from the gates.

Which of the following expressions always represents the distance between the turtle and Achilles during the race?