A 24-year-old patient complains of new growth in the right lateral surface of the neck. The new growth was first noticed 4 months ago, no appointment with doctor was made. Objectively: the face is asymmetrical due to the new growth in the upper third part of the right lateral surface of the neck. Palpation allows to detect the painless rounded new growth sized 3x3,5 cm; its consistency is dence and elastic; the skin abowe the growth has no changes of colour and can be folded. The new growth is not melded with its surrounding tissues and is situated along the front edge of the sternocleidomastoid muscle. Puncture consists of pale yellow viscous fluid. What is the most probable diagnosis?