I suppose the time between the two airports can't be zero,
and each flight duration is no longer than 6 hours,  so the
total 4 flight will take place less than 24 hours, and no
less than 0 hour, Am I right?

> I suppose the time between the two airports can't be
zero,
> and each flight duration is no longer than 6 hours,  so
the
> total 4 flight will take place less than 24 hours, and no
> less than 0 hour, Am I right?

I think, that you not correctly have understood formulation
of task.
4 flights here have no any of relation to formulation of
task.

I have tried my program to solve the case which the whole
duration is no longer than 6 hours, but still got W.A,
maybe output 0 is also a possible answer,