A 43-year-old patient complains about a neoplasm in the right submandibular region that appeared a month ago after angina. Body temperature is up to 37,0 — 37, 2°C. The patient underwent anti-inflammatory therapy but it led to no reduction of the neoplasm. Objectively: in the right submandibular region palpation reveals a slightly painful, spherical, freely movable, well-defined neoplasm of dense and elastic consistency. The duct of submandibular salivary gland discharges transparent saliva. The sublingual plica is unchanged. What is the most likely diagnosis?