Why the focal length of a plane mirror is infinite?

Plane mirror is formed with plane surface so it’s curvature is 0 and radius of curvature is infinite.
f=R/2
f= infinite /2
f= infinite.

Also so position of image and position of object is equal
1/f = 1/v +1/u
1/f = 1/x-1/x
1/f = 0
f = 1/0 = infinite.