A 55-year-old patient consulted a dentist about a roundish tumour-like formation of about 1 cm in diameter located within the vermilion border of his lower lip. Objectively: the tumourlike formation protrudes about 5 mm above the vermilion border, it is dense and grayish-red. The surface of the formation is covered with thin scales that can hardly be removed. What is the most likely diagnosis?