A 17-year-old man complains of a cosmetic defect in form of light spots on the teeth. In the area of his residence the fluoride concentration in drinking water is at the rate of 1 mg/l. Objectively: vestibular surface of the 11,12, 21, 22, and tubercles of the 16, 26, 36 and 46 teeth have chalky spots with shiny surface that have been present since the time of eruption. What is the most likely diagnosis?