Directed Graphs and Network Flows · Hard problem

A flow network has source $s$, sink $t$, and vertices $a,b,c,d$ in between, with capacities $c(s,a)=4$, $c(s,b)=6$, $c(a,b)=1$, $c(a,c)=3$, $c(a,d)=2$, $c(b,c)=2$, $c(b,d)=5$, $c(c,t)=4$, $c(d,t)=5$. What is the maximum flow from $s$ to $t$?