  Using the mapping of the partition function of the two-dimensional Ising
model onto a pfaffian we evaluate the domain wall free energy difference for
the pure and disordered Ising model close to the pure fixed point. Using this
method very large lattices can be studied exactly and we confirm that disorder
even including frustrating interactions indeed are irrelevant close to the pure
fixed point. The finite-size renormalization group flow shows a power-law
behavior modified by a logarithmic term that dominates for small lattice sizes.
