A 54-year-old patient complaining of tumor-like growth on the red border of the left lower lip has made an appointment with a dentist. Examination revealed the affected area to be sized 1х1,5 cm, and have irregular shape with clear margins. Focus of the affected area does not protrude from the surrounding red border; the affected area is greyish-white in colour and covered in thin closely placed scales. What is the most probable diagnosis?