A 7-year-old girl was brought to a hospital for the purpose of oral cavity sanitation. She was born and has been living in an area where fluorine concentration in water makes up 2,5 mg/l. Examination revealed symmetrically placed dark spots on the vestibular surfaces of the 11, 21, 31, 41 teeth as well as on the tubera of the 16, 26, 36, 46 teeth. It is known from the anamnesis that the teeth cut out already with affection. What is the most probable diagnosis?