Program to determine focal length of a spherical mirror

Write a program to determine the focal length of a spherical mirror.

Focal length is the distance between the center of the mirror to the principal foci

In order to determine the focal length of a spherical mirror, we should know the radius of curvature of that mirror. The distance from the vertex to the center of curvature is called the radius of curvature

The focal length is half the radius of curvature
Formula :

F =   ( R / 2 )      for concave mirror
F = - ( R / 2 )      for convex mirror

Examples: 

For a concave mirror

Input: R = 30 
Output: F = 15

For a convex mirror

Input: R = 25
Output: F = - 12.5

To solve the problem follow the below idea:

Use the formula given above to calculate the focal length of a concave/convex spherical mirror

Focal length of sphericalconcave mirror is : 15 units
Focal length of sphericalconvex mirror is : -15 units

Time complexity: O(1) because constant operations are performed.
Auxiliary space: O(1), as constant operations are being performed.