REVERE – Police said a combative robber almost bit a finger off a Revere Street store employee who chased the man after he grabbed a handful of money from the One-Stop convenience store register on Wednesday evening.Mark Gleeson, 43, of 18 Ellsworth Terrace, Lynn, approached the store counter with a soda can and grabbed cash from the register as Sureshkumar Patel rang up the purchase. Patel chased Gleeson as he fled the store and caught up with him near 50 Shurtleff St. where the pair struggled and Gleeson nearly bit off the store worker’s right middle finger.Off-duty Police Sgt. Jeffrey Langone spotted the pair and ran to help Patel. He managed to pin Gleeson on the street until other officers arrived. Police charged Gleeson with unarmed robbery, third offense, mayhem, assault and battery, assault and battery with a dangerous weapon, assault and battery on a police officer, resisting arrest, disorderly person and a civil rights violation involving bodily injury.While being booked at the Revere police station, Gleeson, according to police reports, told police he smoked $200 worth of crack cocaine in Lynn earlier Wednesday and told police he was a military veteran.He was taken from the police station to Whidden Memorial Hospital after complaining of chest pains. Langone also went to Whidden to be examined and treated for foot and shoulder injuries.