"""
Project Euler Problem 28
========================

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 both diagonals is 101.

What is the sum of both diagonals in a 1001 by 1001 spiral formed in the
same way?
"""

sum = 1

term = 1
for i in range(3, 1002,2):
    square = i * i
    sum += square + (square - 2 * term)
    term += 1

term = 1
for i in range(2, 1001, 2):
    square = i * i
    sum += (square + 1) + (square - (2 * term - 1))
    term += 1

print sum
