A 33-year-old patient complains of slowly growing tumor mass in the left submandibular area. Objectively: a tumor with soft elastic consistency can be palpated in the left submandibular area; the tumor changes its configuration when squeezed with fingers and is painless. The skin covering the tumor has no changes in colour, movable. There are no changes detected in the oral cavity. Salivary glands function normaly. Shrinking-inflation symptom is absent. What is the most probable diagnosis?