MATCH (person1:Person {id:"17592186055119"}), (person2:Person {id:"8796093025131"})
OPTIONAL MATCH (person1)-bfs[r](a, b | type(a) = "KNOWS", 15)-(person2)
RETURN
CASE r IS NULL
  WHEN true THEN -1
  ELSE size(r) + 1
END AS pathLength;