Need to populate first five digits from a number field to another number field

Need to populate first five digits from a number field to another number field

Hi,

I have a Product Serial number field and a Product Batch number field in my form. Batch Number is the first five digits of the Product Serial No.
User should either enter the Product Serial No. and Batch Number should automatically be populated in the Product Batch Number field or if the user does not have the Product Serial No., he should be able to enter the Batch No. directly in the Batch No. field.

Can someone help?

Thanks