Updating the nos that starts with 2 alone

Hello,

My request seems to be simple but I am unable to do...

I have two columns ... Column B and Column D.

Column B is a mix of numbers that starts with 2,1,5
In column D I require to update the numbers that starts with 2 alone until the next number starts.

Logically this is how it works....

All the numbers that starts with 2 is the mother and others are its child
so for each child (not starting with 2) the mother number needs to be updated

Please provide me the macro which makes the job simpler.
I am attaching the doc for ur reference

Thank U.

You can do this with a formula. D2 and down:

=IF(VALUE(LEFT(B2,1))=2,"",IF(E1="",B1,D1))