Given a matrix of shape N×N arranged in a "spiral", with its numbers spiralling from 1 to N² inward, what is the sum of its diagonals? See examples to clarify what a spiral is.

Example 1: The input: 3 Gives the following spiral:

1 2 3 8 9 4 7 6 5

The sum of the diagonals is: 1 + 3 + 5 + 7 + 9 = 25

Example 2: The input: 4 Gives the following spiral:

1 2 3 4 12 1314 5 11 1615 6 10 9 8 7

The sum of the diagonals is: 1 + 4 + 7 + 10 + 13 + 14 + 15 + 16 = 80