How do I get flash in pokemon Fire Red?

in fire red, I know you get flash on route two, but in order to go through the path I need cut, I get cut on the ss anne, but I cant find a way to backtrace out of Cerulean City because of the ledges, I cant get any farther in the game because I cant get through the caves, how do I obtain flash and get out of Cerulean City successfully when I get cut??