Directed Graphs and Network Flows · Medium problem

A digraph has vertices $1,\dots,6$ and arcs $1\to2$, $2\to3$, $3\to1$, $3\to4$, $4\to5$, $5\to4$, $5\to6$. Out of all $\binom{6}{2}=15$ pairs of vertices, how many pairs are NOT mutually reachable?