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 fff meters from the gates, while the turtle is only ggg meters from the gates. Which of the following expressions always represents the distance between the turtle and Achilles during the race?