Directed Graphs and Network Flows · Superchallenge problem

A digraph has two separate pieces: $\{1,2,3\}$ with arcs $1\to2$, $2\to3$ only (no arc $3\to1$), and $\{4,5,6\}$ with arcs $4\to5$, $5\to6$ only (no arc $6\to4$) – no arcs at all between the two pieces. What is the minimum number of new arcs that must be added so the whole digraph has an Eulerian circuit?