I suspect I am in the correct forum. However, due to partial or complete ignorance on my part, of disciplines above and including algebra, I may not be.

So in an effort save some time for this particular situation, I am presenting this here, before I embark on the learning curve,...if that is even possible.

Trying to define the equation I would use to find the value of a given number, using the following logic:

Given Number: 2 its value is: 1 determined via 1

Given Number: 3 its value is: 3 determined via 1+2

Given Number: 4 its value is: 6 determined via 1+2+3

Given Number: 5 its value is: 10 determined via 1+2+3+4

... to infinity

This feels obvious,...but I just can't seem to see it.