  We propose a one-parametric extension of the Starobinsky $R+R^2$ model by
adding the $(R+m^2\beta^2)^{3/2}$ term. The parameter $m$ is the inflaton mass,
which is determined in the same way as in the Starobinsky model, and $\beta$ is
a dimensionless constant. Using the Einstein frame and the scalar field
potential, we get the inflationary parameters of the model proposed. The value
of the tensor-to-scalar ratio $r$ can be significantly larger than in the
Starobinsky model. The considered inflationary model is in a good agreement
with the current observational data. The corresponding scalar field potential
is a polynomial of the exponential function.
