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https://d-osin.com/2013/07/08/%d1%82%d1%80%d0%b8-%d1%81%d1%82%d0%be%d1%80%d0%be%d0%bd%d1%8b-%d1%81%d0%b5%d1%80%d0%b2%d0%b8%d1%81%d0%b0/
Три стороны сервиса