Directed Graphs and Network Flows · Hard problem

A network has capacities $c(s,a)=4$, $c(s,b)=2$, $c(a,b)=3$, $c(a,t)=1$, $c(b,t)=5$, starting with zero flow. Push the augmenting path $s\to a\to b\to t$ as far as it goes. What is then the residual capacity of the backward arc $b\to a$?