Please consider the Sample input and output :
  The 1st flight ( connects 1 and 2 ) and the 2-nd flight ( 2-3)
are flights that depart from one airport ( airport 2 ) ( The flight
are 2 directions flight ) . But the sample output indicates
that their numbers are 4 and 2 , so their greatest common divisor
can't be 1 as the problem description !
   I don't know if I wrong . I got WA too . Please explain for me
Thanks a lot !

 there is another flight from airport 2, it is (2, 4) and according
to the sample output it is numbered 3. We know that gcd(2, 4, 3) = 1.
It's all OK.