# Retrieve the independent variables from a linear replacement list

Is there a good way to convert a linear replacement list into a list of the independent variables? For example, I would like to take {n1->n3+n4, n2->n3+2n4} and get {n3,n4}, the variables that n1 and n2 are expressed in terms of.

lst = {n1 -> n3 + n4, n2 -> n3 + 2 n4};