Thanks to a teenager's willingness to fight back and a dropped check card, a habitual violent criminal is just beginning to serve a long prison sentence for a Carver County ­convenience store robbery.

Demetrius Derden, 28, of Blaine, was sentenced in federal court in St. Paul to 14 years and two months in prison for the armed robbery in December 2011 of the Mayer Oil Co. station on Ash Avenue.

Court documents reveal that Derden had all he could bargain for when he confronted the clerk over a forgotten safe combination, at one point forcing the teen to his knees and putting the barrel of what turned out to be a BB gun, to the high school senior's head.

"You'd better remember [the combination]," Derden demanded. "Your life depends on it."

The clerk begged for his life and then heard the gun click but not fire. It was then that the teen challenged the robber, tearing off his mask and suffering a bitten finger in the confrontation.

Derden grabbed the money and fled, leaving behind an empty bottle of rum — as well as a check card with his name on it. Authorities needed just hours to arrest Derden at his home.

"The clerk ... believed he was about to be shot dead," the Carver County attorney's office said in a statement Tuesday announcing the sentencing. "The clerk then stood and began the fight of his life."

Authorities have not said how much money Derden came away with, but his sentence includes making restitution of more than $9,000. The clerk's name also has not been disclosed.

According to prosecutors:

Derden, wearing a mask, entered the store and pointed the gun at the clerk, who was behind the cash register. The teen followed orders and put all the money from the register — more than $2,000 — into a plastic bag.

The clerk was then ordered to a back room and told to empty the store's two safes. The teen told Derden he didn't know the combination to the second safe. That's when Derden had the teen kneeling and pleading for mercy.

Upon hearing the gun's click, the clerk "seized this as an opportunity" to stand up, prosecutors wrote in one court filing. Derden pushed him back down and struck him on the head with the gun.

The clerk was treated at a hospital for his injuries. "To this day, [the teen] has several permanent scars on his head and face," the prosecution wrote.

In arguing for nearly 16 years in prison, prosecutors called Derden "a dangerous repeat offender who had committed crime after crime for the past 10 years, regardless of whether he was on probation, supervised release or had just been released from prison."

His criminal history in Minnesota includes convictions for robbing a woman of her purse and punching her in the face, receiving a stolen firearm, and twice robbing a gas station in Blaine.

The defense countered that five years in prison would be sufficient, noting that Derden pleaded guilty and has cooperated with authorities. The defense added that Derden had a violent upbringing in Chicago, was the product of a broken home and that his twin 7-year-old daughters "will not see their father while he is in prison."