Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:
21 22 23 24 25
20 7 8 9 10
19 6 1 2 11
18 5 4 3 12
17 16 15 14 13
It can be verified that the sum of the numbers on the diagonals is 101.
What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?
Start with 1, add 2 each time to get the next four numbers, then
add 4 each time to get the next four numbers, then add 6 each time
for the next four numbers, then add 8 each time for the next four
numbers, etc. until one of the numbers added is 1001 squared.
...
Well, that was pretty straightforward.