Dynamic Formula Help

[Sucram1]

This seems as though it should be simple but I am not getting it.

1. Category Names are Data Validation lists.
2. If user selects Subcategory from the Data Validation list that cell becomes a Subcategory of the category in the cell above.
3. If Subcategory chosen, Sum Total to automatically calculate and move down to the corresponding Subcategory row below.

Question: What formula should I put in the Sum Total column so the cells automatically calculate?

Thank you.