##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?


def fi(n):
    trinow = 1
    result = 1
    for i in range(3, 2 * n, 2):
        trilast = trinow
        trinow = i * i
        p = (trinow - trilast) // 4
        result = result + 4 * trilast + 10 * p
    return result


print(fi(501))
