Directed Graphs and Network Flows · Superchallenge problem

A network has $s,a,b,c,d,e,t$ with links $sa,sb,sc,ad,bd,be,ce,dt,et,ab,cd$, and each of $a,b,c,d,e$ may be used by at most ONE path from $s$ to $t$. What is the maximum number of internally vertex-disjoint paths from $s$ to $t$?