a, b, c, d are non-zero reals and f(x) = (ax + b)/(cx + d). If f(-1) = 0, f(1) = 1 and f(f(x)) = x for all x (except -d/c). The unique value which is not in the range of f(x) is

1) 1/3
2) 2
3) 0
4) -1/2