(a) A person is guilty of robbery in the first degree when, in the course of the commission of the crime of robbery as defined in section 53a-133 or of immediate flight therefrom, he or another participant in the crime: (1) Causes serious physical injury to any person who is not a participant in the crime; or (2) is armed with a deadly weapon; or (3) uses or threatens the use of a dangerous instrument; or (4) displays or threatens the use of what he represents by his words or conduct to be a pistol, revolver, rifle, shotgun, machine gun or other firearm, except that in any prosecution under this subdivision, it is an affirmative defense that such pistol, revolver, rifle, shotgun, machine gun or other firearm was not a weapon from which a shot could be discharged. Nothing contained in this subdivision shall constitute a defense to a prosecution for, or preclude a conviction of, robbery in the second degree, robbery in the third degree or any other crime.
(b) Robbery in the first degree is a class B felony provided any person found guilty under subdivision (2) of subsection (a) shall be sentenced to a term of imprisonment of which five years of the sentence imposed may not be suspended or reduced by the court.