arc(a,b,1).
arc(b,f,2).
arc(a,f,4).
arc(b,e,4).
arc(f,e,1).
arc(f,a,3).

chemin(X,Y,C) :-
    arc(X,Y,C).

chemin(X,Y,C):-
    X \=Y,
    chemin(X,Z,A),
    arc(Z,Y,B),
    C is A+B.