Heist

You are escaping from the police after stealing a famous painting. The path to the exit is linear as you are in a corridor. We'll represent it as a line on the $x$ axis. You are standing at $x=0$, and the exit is at $x=D$. However your escape has been complicated by the following situation. Several sections of laser beams block your path. Each section begins at $x=a$, and ends where another secti