Graph Algorithms · Medium problem

A graph has vertices $k,l,m,n,o,p,q$ and edges $kl, km, ln, mn, mo, np, op, pq, oq$. Running a DFS from $k$ that visits each vertex's unvisited neighbors in alphabetical order discovers a back edge connecting $n$ and $p$. What is the length (number of edges) of the cycle that this back edge forms with the DFS tree path between $n$ and $p$?