what's different between undefined and 3?
can you help me?

Isenbaev -> a -> b -> c.
In this chain, a is 1, b is 2 and c is 3. Obviously,there is a path from Isenbaev to c.

But In this situation:
Isenbaev -> a -> b.
c.
c isn't a teammate of anyone who is connected with Isenbaev. So, c is 'undefined'.