Field creation based on selection

[AmCh]

Hi all,

I have for example the following field:

Field_Input

A1

A2

B1

B2

C1

The enduser would select e.g. A2 and B1. selection A2 should give 12, B1 should give 21.

Based on his selection I want to store the values in a new field:

Field_Output

12

21

Any help please?

Thanks in advance.