SUM function problem / discussion

Im comming from a long way to get the following column
Now i want to sum over the column B but what numbers I want to sum depends of an input number X so
if cel D1 is 6 i want to sum B1till B6 and if D1 = 9 i want to do the sum over B1 till B9 etc
A B
1 0,999829785
2 0,961374793
3 0,924398839
4 0,888845038
5 0,85465869
6 0,821787202
7 0,790180002
8 0,759788464
9 0,73056583
...
...

I am just curious if there is some sort of condition that does the job
An alternative solution would be to make a third column with incremental values of B and use D1 as lookup value in a vlookup function with 3 as column index
(I actually got this idea when i was writing this problem)

1 0,999829785 0,999829785
2 0,961374793 1,961204578
3 0,924398839 2,885603417
4 0,888845038 3,774448455
5 0,85465869 4,629107145
6 0,821787202 5,450894347
7 0,790180002 6,24107435
8 0,759788464 7,000862813
9 0,73056583 7,731428643

=VLOOKUP($D$1;A:C;3;FALSE)

Try this:

SUM(OFFSET($B$1,0,0,D1))

Tnx guys, this is great learning material!
from thinking in the start it might not be possible to seeing 4 ways how to do it !