The girl of 11 y.o. She is ill for 1 month. She has "butterflytype rash on face (spots and papules), pain and swelling of small joints on arms and legs, signs of stomatitis (small-sized ulcers in mouth). CBC: Hb- 80 g/L, RBC- 2,9 × 1012/l, WBC- 15 × 109/l, ESR - 40 mm/hour. Urinalysis: protein- 0,33 g/L. What is the most probable diagnosis?