UDF to generate sequential number

Hey All,

I am working with INVOIC01 IDOC and need some help in mapping.

My structure will contain multiple E1EDP05 under E1EDP01

E1EDP01

E1EDP05

E1EDP05

E1EDP01

E1EDP05

E1EDP05

E1EDP05

On the receiver side, I need to generate sequential number like below:

The UDF I have written is below but it doesn't seem to work:

Can some please help me with the correct UDF

Thanks