  In this paper, we investigate the action of pseudogroup of all point
transformations on the natural bundle of equations $y'' = u^0(x,y) + u^1(x,y)y'
+ u^2(x,y)(y')^2 + u^3(x,y)(y')^3$. We calculate the 1-st nontrivial
differential invariant of this action. It is a horizontal differential 2-form
with values in some algebra, it is defined on the bundle of 2--jets of sections
of the considered bundle. We prove that this form is a unique obstruction to
linearizability of these equations by point transformations.
