A 20-year-old patient complains of a tumour-like formation on the lateral surface of his neck on the right. He revealed this tumour a year ago. Objectively: there is a semi-oval well-defined tumour 3х2 cm large on the lateral neck surface. The tumour doesn’t hold together with skin, it is painless, dense and elastic, upwardly and sidewardly movable. Regional lymph nodes are not enlarged. After the tumour puncture some light yellow liquid was obtained that consisted of cast-off epithelium, cholesterol crystals, lymphocytes, erythrocytes. What is the most probable diagnosis?