There is a theory that all Slavs descended from western Ukraine. Although, there was a Slavic tribe known as White Croats in Transcarpathia. Slavic I2 may have gone through bottled neck in the Balkans. It’s origin maybe somewhere in central Europe or close to eastern Europe. I don’t know.
All of N1c1 marker Belarusians have belongs to the so-called “South Baltic” branch marked by N1c1-M2783 marker. The marker is closely related to the Scandinavian branch marked by N1c1-L1025. See the tree below.
Lithuanians, Poles, Latvians, Ukrainians have the same branch marked by N1c1-M2783
Russian have all kinds of N1c1 markers – Finnish, Uralic, South-Baltic, Yakutian.
Finns have their own branches.
Estonians have both South-Baltic and Finnish
The split between Finnish and South Baltic branches of N1c1 occured around 3,400 years ago at N1c1-Vl29.