Mordvins live further away from us than Greeks or Italians or Germans. Belarusians are mixed with Lithuanians. If you study about origins of Slavs you will find the Balts are ancestors of original Slavs. If you want to find a hardcore Slav that lived 2,500 years ago, then look at Lithuanian villager. Baltic ancestry among Belarusians, north-western Russians, north-eastern Poles makes us more Slavic originally.
I am not exaggerating about proximity between Balts and Slavs