M is a finite set of points in a plane. Point O in the plane is called an “almost centre of symmetry” of set M if it is possible to remove from M one point in such a way that among the remaining members O is the centre of symmetry in the usual sense. How many such “almost centres of symmetry” may a finite point set in a plane have? Indicate all such points.(V Prasolov, Moscow) symmetrygeometrygeometric transformation