Table Of Calculations Down To A Single Array Formula

I'm trying to condense a table of calculations down to a single array formula, but am getting stuck on one piece of it.

The table data is very simple, and can be in just two columns:

1, value
2, value
3, value
4, value
5, value

Where 1 thru 5 are time periods, and values are various numbers.

I want to perform the GammaDist function on each value. It requires a time period input, and in this case it is the 1 thru 5 in the table. So at the end of period 5, the formula for the first value would be

=Gammadist(5,x,y,TRUE) (x,y values not important here)

and the formula for the second value would be:

=Gammadist(4,x,y,TRUE)

The only thing that changes is the period number.

So, my goal is to write an array formula that will sum the GammaDist for each of the 5 rows, for all timeperiods (which is 5 in this case)

I can get this far:

={SUM(B1:B5*GAMMADIST(ROWS(1:5)-1,C1,C2,TRUE))}

But this passes 1 thru 5 to all rows, I only want 1 thru 5 passed to row 1, 1 to 4 passed to row 2, etc.