Calculation of amount between dates without using customer exit..

My report need to calculate the total amount for the dates

User will provide a date and a number like 5,10.. difference to calculate the remaining dates.

i used formula to calculate the next two dates

ex- user entry(U) -: 20-03-14

formula 1 (F1)-: U-5 = 15-03-14

formula 2 (F2)-: U-10 = 10-03-14

DATA-

dates amount

10-03-14: 100

11-03-14: 100

.

.

15-03-14: 100

.

.

20-03-14: 200

assume there is lost of data in b/w

i want to calculate that without using variable