Baltic States + Russia: Russia has a mains voltage of 220 volts. In trains and some hotel bathrooms Russia has a mains voltage of 110 volts. Sockets are Standard European 2 pin although some thick plugs can cause problems. A reducer or world plug may be convenient, an adaptor is not necessary.
Estonia, Latvia and Lithuania have a mains voltage of 220 volts. There can be brief moments of power outages and fluctuations in amperage; an adaptor is useful then. You can use the European plug everywhere.