Funding: the study received a grant #VG08/2020 supported by Vernadsky Crimean Federal University.

Compliance with ethical standards: the study was approved by the Vernandsky Crimean Federal University ethics committee (Minutes #6 of June 04, 2020). Parents of the children submitted the informed consent forms allowing their children to participate in the experiment.

Author contribution: Kaida AI — data collection and processing, article authoring; Eismont EV — research planning, data collection, article authoring; Mikhailova AA — data processing, article authoring; Pavlenko VB — research planning, article authoring.