Euler Project 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?

My Solution

(defun Euler28(n)

  (loop for i from 3 to n by 2

          sum(let ((squarenum (* i i))

                      (tempnum (- i 1)))

                     (+ squarenum (- squarenum tempnum) (- squarenum (* 2 tempnum)) (- squarenum (* 3 tempnum))))))