Properties of Flow: f(v,w) -- flow on edge (v,w)
0<= f(v,w) <= c(v,w) : the flow through an edge cannot exceed the capacity of an edge
for all v except s,t S u f(u,v) = S w f(v,w): the total flow entering a vertex is equal to total flow exiting vertex
total flow leaving s = total flow entering t.
Notation on edges f(v,w)/c(v,w)