SAUGUS – A 16-year-old Melrose boy was arrested within minutes after he robbed the Kristos Mini Mart at knifepoint on Wednesday, police said.The unidentified juvenile was charged with armed robbery and armed assault with intent to rob.At about 6:05 p.m., several police units responded to the store at 465 Essex St. after a male wearing an orange hooded sweatshirt allegedly robbed the store of $300, according to police.The suspect allegedly used a knife between 10 and 12 inches long and told the store clerk, “Open the drawer and give me the money.”Two other youths wearing hooded sweatshirts were seen near the suspect shortly after the robbery, but police did not have enough evidence to charge them with a crime.Police said only one employee was inside the store at the time of the robbery but two other people witnessed the crime from outside.