A 12-year-old boy complains about pain and swelling in the parotidomasticatory area on the left, body temperature rise up to 37,5°C. He has been suffering from this for 5 years. Objectively: palpation reveals a dense painful nonmobile formation 3,5х5 large in the parotidomasticatory area on the left. Skin colour is unchanged. Orifice of the left salivary gland duct excretes transparent secretion. What is the most probable diagnosis?