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 main diagonals in a N by N spiral formed in the same way?
Source: Project Euler