Mapping issue

Hi Experts,

I am working on a scenario, where I need to pass a value to the target field validating if the value is not in the first segment in the IDOC then need to pass the value from the segment. Scenario is Invoice02 IDOC to File.

Condition : 'E1EDK03 001 - DATUM, if none, then to get E1EDK03 026 - DATUM.

Can someone help me to do a logic?

Thanks in Advance!!