A 37-year-old woman complains of a white formation on the mucosa of her upper lip. The formation was noticed accidentally. Objectively, on the teeth of her lower jaw there are 2 dental bridges, the crown of tooth 21 is half-destroyed, the tooth is untreated. In the projection of tooth 21 on the labial mucosa, there is an irregularly- shaped white plaque 4x9 mm in size with clear margins. The plaque is slightly raised above the mucosa, cannot be scraped off, painless, and without an infiltration in its base. The regional lymph nodes cannot be palpated. Make the diagnosis: