Creating A Input Or Output Field

[skoz55]

How could I make a field accept inputs or produce an output.

Ex.
Two fields that produce either radius or area of a circle depending on what the user inputs.

So if the user inputs the radius of a circle in A1, then B1 automatically outputs the area of the circle. If the user inputs the area of the circle in B1, then A1 automatically outputs the radius of the circle. (The formula and equation aspect is not a problem).