Pardon the typo, it should be Russia instead of Russian
/u/kieslowskifan has previously answered:
Why does Russia control that little chunk of land between Lithuania and Poland?
What happened to the Germans in areas such as Kaliningrad and Western Poland?
/u/kaiser_matias contributed to the thread above and How did Kaliningrad remain a Russian territory?
See also Why did the USSR annex Kaliningrad after WW2? by /u/Augenis