Directed Graphs and Network Flows · Hard problem

A flow network has source $s$, sink $t$, and vertices $a,b,c,d$, with capacities $c(s,a)=6$, $c(s,b)=4$, $c(a,c)=5$, $c(b,c)=2$, $c(b,d)=3$, $c(c,t)=4$, $c(d,t)=5$, $c(a,d)=1$. What is the minimum possible capacity of an $s$-$t$ cut in this network?