A 42-year-old patient complains about gingival pain, progressing gingival hemorrhage, increasing tooth mobility, halitosis. Objectively: gums are evidently hyperaemic, extremely edematic, they bleed easily on palpation. Tooth roots are exposed, parodontal pouches are 4-6 mm deep, and contain purulent exudate, there is also supragingival and subgingival dental calculus. II-III grade tooth mobility is present. Orthopantomogram shows resorption of interdental septa down to 1/2 of their height. What is the most likely diagnosis?