A 54-year-old patient complains of a tumor-like formation in the region of his lower lip which appeared 1-1,5 months ago. Objectively: there is a round tumourlike formation up to 1cm in diameter on the red border of the lower lip on the right. Roll-like edges of the formation protrude above the level of the red border. In the centre of the formation a brown crust is visible. After its removal a crateriform hole can be seen. There is no bleeding or pain. Regional lymph nodes are not enlarged. What is the most likely diagnosis?