Parents of a 12-year-old child are concerned about the child having white spots on the frontal teeth of the upper jaw; the spots appeared half a year ago. Objectively: there are chalky spots detected in the cervical zone of the 11th, 12th, 13th, 21st, 22nd, 23rd teeth vestibular surface. The enamel in those spots is dull; probing revealed it to be pliant and coarse. The anamnesis states short-time pain caused by chemical stimuli. What is the provisional diagnosis?