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"""

Problem 28

Url: https://projecteuler.net/problem=28

Statement:

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?

"""

from math import ceil

def solution(n: int = 1001) -> int:

"""Returns the sum of the numbers on the diagonals in a n by n spiral

formed in the same way.

>>> solution(1001)

669171001

>>> solution(500)

82959497

>>> solution(100)

651897

>>> solution(50)

79697

>>> solution(10)

537

"""

total = 1

for i in range(1, int(ceil(n / 2.0))):

odd = 2 * i + 1

even = 2 * i

total = total + 4 * odd ** 2 - 6 * even

return total

if __name__ == "__main__":

import sys

if len(sys.argv) == 1:

print(solution())

else:

try:

n = int(sys.argv[1])

print(solution(n))

except ValueError:

print("Invalid entry - please enter a number")