  Exact ground states are calculated for the Sherrington-Kirkpatrick (SK)
spin-glass containing up to N=90 spins. A ground-state energy per spin
$e^{\infty}_0 = - 0.7637 \pm 0.0004$ is found from the $N$ dependence of the
misfit parameter, which is a measure of frustration of the system. The results
are compared with those of two related models, which can be introduced by
replacing all interactions of the SK model by ferromagnetic or
antiferromagnetic ones of the same strengths. A parameter $x$ is introduced,
which describes the fraction of antiferromagnetic interactions in these types
of models. From the $x$ dependence of finite models it is concluded, that the
the SK model $(x=0.5)$ assigns a transition between a ferromagnetic state
$(x<0.5)$ to a spin-glass state $(x>0.5)$.
