How to keep variable constant with other one changing. (Apartment Numbers )

[Avetis]

Hello Everyone. I am going crazy to find the answer.

So I am working with address data and I have buildings with over 100 apartments. I have current data for up to 50 apartment. Therefore I want to continue numbering and letters for up to available units.

Example
The building has 50 Units
I have first 20 Units
-1A
-1B
-1C
-2A
-2B
-2C
Each Floors changes Number and letters start again

What I need is to continue
-3A
-3B
-3C
-4A
-4B
-4C

Thank you so much for your help

 

[trucyeetung]

Hi, you key in as example of this picture, the formular just using AND FORMULAR &
Best regards,