A 15-year-old girl complains of brief pain attacks in her teeth due to chemical stimuli. Objectively: on the contact surfaces of the 11, 21, 22 teeth there are enamel areas matt white in color, with lost shine, covered in large amount of dental deposit. Enamel is softened and can be easily chipped off with excavator. Probing of lesions is painless. Percussion is painless. No reaction to cold stimuli. Make the diagnosis: