I have a table which stores Hierarchy of Customers i.e., GrandParent, Parent and Child as follows

CustomerNum     Amount  GrantParent   Parent           
-----------     ------  -----------   ------ 
8046026507      100     NULL          1872539355
8099032159      100     1872539355    8046026507
1872539355      100     NULL          NULL

I need a SQL query to get the Sum of AMOUNT when I pass a Customer based on the Hierarchy.
For Example When I pass customer ‘1872539355’(GrandParent) I should get ‘300’ (Sum of all parent, child) of that customer. When I pass ‘8046026507’(Parent) I should get ‘200’. When I pass 8099032159(Child) I should get ‘100’. One Grand Parent Can have multiple Parents and One Parent Can have multiple childs.

Many thanks in Advance.