A 16-year-old patient complains about a cosmetic defect in form of white spots in the region of the upper frontal teeth. The defect was revealed long ago and doesn't change with time. Objectively: white spots on the vestibular surfaces of the 11,12,21, 22 teeth close to the cutting edge and on the vestibular surfaces of the 16,26,36,46 teeth close to the masticatory surface. On probing the spot surface was smooth, painless; cold stimulus produced no pain. The spots couldn't be stained with 2% solution of methylene blue. What is the most likely diagnosis?