A 5-grade pupil complains about extensive skin rash accompanied by intensive itch, especially at night. Objectively: there are small red papules set mostly in pairs in the region of interdigital folds on both hands, on the flexor surface of radicarpal articulations, abdomen and buttock skin as well as internal surface of thighs. In the centre of some papules vesicles or serohaemorrhagic crusts can be seen. There are multiple excoriations. What is the most likely diagnosis?