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?

My solution in Ruby:

corners = [1]
num = 1
add = 2
500.times do
  4.times do
    num += add
    corners << num
  end
  add += 2
end
sum = 0
corners.each { |c| sum += c }
puts sum