A 20-year-old male patient complains of acute pain during eating, bad breath, general weakness. Objectively: interdental papillae and marginal gingiva are covered with dirty gray film, any touch is painful. The body temperature is of 37,5°C. Blood count: RBC - 4,8 × 1012/l, Hb- 150 g/l, colour index - 0,9, WBC -10,5 × 109/l, JG - 2%, BASO - 1%, EOS -5%, segmented neut. - 8%, stabs - 47%, LYM - 35%, PLT - 250 × 109/l. ESR - 20 mm/h. What is the most likely diagnosis?