A 24-year-old patient came to the dentist complaining of chalky lesions on the front teeth. Objectively teeth 11, 21, 31, 41 present with chalky lesions separated by areas of healthy unchanged enamel. Lesion surface is coarse; there is no response to thermal stimuli. Childhood years of the patient were spent in the area with fluorine level of 1.8 mg/L in drinking water. Make the diagnosis: