A 10,5-year-old child complains of painful rash on his lips. Objectively: the red border of the lips is swollen, hyperemic, covered in fissures and numerous scabs of dried blood. The skin of the upper lip has small blisters containing serous substance, which merge with each other in some places. Maceration and weeping skin also can be observed, especially in the corners of the mouth. What is the most likely diagnosis?