Could anyone please give me any suggestion on how to code following condition
in excel?

In colume A, it displays the date for activities.

In colume B, it displays a list of activities for everyday, such as input
products, output products and keep inventories. The number of activities is
not fixed, one day could performance 6 activities, but on the other days, it
could performance 9 activities, but a blank row must be inserted between two
days for separation.
the activities of "keep inventories" will only be performed once at the end
for each day, which will sum up all the quantity of products for today only.

In colume C, it displays the number of quantity for products.

Does anyone know how to code the condition in excel to sum up all the number
of quantity from above until it meets a blank row? please see below example
for detail description.

On 25 Sep, 2006, it contains a lists of activities as show below

[row 1] [a blank row was inserted]
[row 2] 25 Sep input products 10
[row 3] 25 Sep output products -5
[row 4] 25 Sep input products 7
[row 5] 25 Sep input products 6
[row 6] 25 Sep output products -15
[row 7] 25 Sep keep inventories 3 = 10-5+7+6-15, which sum up the available
inventories for today only.
[row 8] [a blank row was inserted]
[row 9] 26 Sep input products 20
[row 10] 26 Sep output products -15
[row 11] 26 Sep input products 8
[row 12] 26 Sep input products 3
[row 10] 26 Sep output products -10
[row 11] 26 Sep input products 6
[row 10] 26 Sep output products -5
[row 11] 26 Sep input products 4
[row 13] 26 Sep keep inventories 11 = 20-15+8+3-10+6-5+4, which sum up the
available inventories for today only.
[row 14] [a blank row was inserted]

Does anyone know how to code this situation in excel?
Thank you for any suggestion
Eric