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No spreadsheet required...
Imagine a triangle that's got rows of 1, 2, 3, 4 items:-
o
o o
o o o
o o o oNow imagine another triangle upside down that "fits" on top,
o x x x x
o o x x x
o o o x x
o o o o xThis makes a rectangle of 4 x 5
So to add up 1,2,3,4 you can do 4 * (4+1) / 2 = 4 * 5 / 2 = 10.
In general that's n * (n+1) /2
So for 31 you get 31 * 32 / 2 = 496km.
I made a spreadsheet earlier and its 496km ;')