numWalls([], 0).
numWalls(['w'|Tail], W) :-
    numWalls(Tail, W1),
    W is W1 + 1.
numWalls([H|Tail], W):-
    H \= w,
    numWalls(Tail, W).
:- numWalls([w,w,n,n,w,w,n,w,n,n,n,w], N), print(N).