  We succeed to find compact analytical expressions which allow to easily
extract the black hole spin from observations of its shadow, without need to
construct or model the entire curve of the shadow. The deformation of Kerr
black hole shadow can be characterised in a simple way by oblateness (the ratio
of the horizontal and vertical angular diameters which are supposed to be
measured by an observer). The deformation is significant in case the black hole
is nearly extreme and observer is not so far from the equatorial plane. In this
approximation, we present: (i) the spin lower limit via oblateness, (ii) the
spin via oblateness and viewing angle, in case the latter is known from other
observations.
