Sorry, wrong answer.
Again, real quantities have + values and virtual quantities have - values. A convex ( spherical mirror has its center on its right side where the reflected light does NOT really go after reflection.
Rays parallel and close to the axis diverge after reflection as if from a virtual focal point behind the mirror.
Also, since f = R/2, f and R must have the same sign.
Try again, please. Click here to return.