A girl is 13 years old. She lives in an area where fluoride concentration in the drinking water is at the rate of 1,6 mg/l. Dental examination revealed some chalklike spots on the vestibular surfaces of all her teeth. The white coloration is more intense in the cenre and less on the periphery. There is light-brown pigmentation in the region of the central incisors along the cutting edge. What is the most likely diagnosis?