Not sure about one thing here, and without giving it away (others should scoop the Merit
reward), I can’t make out why the "-1" is there nor whether it is meant to be included in the summation or rather subtracted after it (the latter I believe, since it lacks brackets). I wouldn’t have thought it (the "-1") to be part of the original formula, but I may be wrong.
Note: If anyone wants to perform the calculus, you can easily do it on Excel (start with i=3 for a reason).
<...> It's about the meaning of subtracting 1 from the equation where i=3, not the calculation. And finding a "string" ...
I know, that's why I said "wants to" as opposed to saying it in a "needs to" context.