Probably an easy answer

I've been trying t figure out how to add sequential numbers to one another without having to make a list of numbers in a bunch of cells. For instance, in A1 I want 1 and A2 I want 25. How can I make A3 the sum of 1+2+3+....+25? The end goal is to be able to change the value in A2 to any whole number and have A3 calculate the result. I have tried SERIESSUM to no avail (most likely my own lack of understanding of how it works) so any help would be greatly appreciated.

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