Directed Graphs and Network Flows · Medium problem

A network has capacities $c(s,a)=5$, $c(s,b)=3$, $c(a,b)=2$, $c(a,t)=4$, $c(b,t)=6$. A flow sends $f(s,a)=5$, $f(s,b)=1$, $f(a,b)=2$, $f(a,t)=3$, $f(b,t)=3$. What is its value?