reys(kiev,odessa).
reys(odessa,lviv).
reys(lviv,kharkiv).
reys(kiev,lviv).
reys(kharkiv,moscow).

not(A):-A,!,fail.
not(_).

avia1(A,B):-reys(A,C),reys(C,B),not(reys(A,B)).

avia2(A,B):-reys(A,C),reys(C,E),reys(E,B),not(avia1(A,B)).