MathNet 2014 Question 19

The graph of the parabola $y = x^2$ is drawn on the Cartesian plane $Oxy$. A triangle $ABC$ is inscribed in the parabola so that its side $AB$ is parallel to the $Ox$ axis and point $C$ lies between the line $AB$ and the $Ox$ axis. It is known that the length of the side $AB$ is 1 less than the length of the altitude $CH$ of the triangle $ABC$.
Find the value of the angle $ACB$.