Directed Graphs and Network Flows · Challenge problem

In a network with arcs $s\to a$, $s\to b$, $a\to b$, $a\to t$, $b\to t$, a flow has $f(s,a)=5$, $f(s,b)=2$ and $f(a,t)=4$. Conservation then forces $f(a,b)$ and $f(b,t)$. What is $f(b,t)$?