"""
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 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?
"""
import time


def spiral_sum(n):
    if n == 1:
        return 1
    else:
        return spiral_sum(n - 2) + 4 * n ** 2 - 6 * n + 6


t0 = time.clock()
print(spiral_sum(1001))
t1 = time.clock()
print("{0} ms".format((t1 - t0) * 1000))
