A 44-year-old patient complains about the astringent sensation in the upper jaw incisors, which appeared 1,5 years ago. Objectively: the most convex part of the vestibular surfaces of the 12, 11, 21, 22 crowns has roundish defects with smooth, shiny, dense surface, that reach dentine in depth. The depth of defects is gradually decreasing from the centre to the periphery. What is the most likely diagnosis?