Year to month and back - entry formula to itself

Hi all,
I am trying to create a simple calculator where I would like to be able to choose between entry to "A1" - year, or "A2" months. I want it recalculate the other one no matter where I put it.
So, I enter 5 years, it will calculate 5*12 in months. If I enter 24 months, it will calculate 24 / 12 in year.

Do you have any idea how to do it?
(so far, if you tell me how to make a macro which start when value is change MANUALLY, not by VBA, I can do it.)

Thanks for any idea.
Olwy

Exacly. That is better than perfect. :-) Thanks a lot, "shukria" or धन्यवाद

Not at all, I just have been in Hyderabad and Lucknow for couple months, but I cannot read it at all. I just love Hindustan and know a little bit of hindi.
Thanks for your help.