Two young men were shot Sunday evening just as a Brooklyn Puerto Rican Day Parade came to a close, cops said.  A 19-year-old man was shot in the left arm and a 20-year-old man took a bullet to the back during a clash on 45th Street near 5th Avenue in Sunset Park just after 7 p.m., police said.  Footage obtained by News 12 showed a shooter dressed in all black opening fire while chasing a man wearing red pants down 45th Street, with the latter then falling behind a parked car. Police haven’t