The Empty Square Sequence

In the image below, squares are either empty or filled with a circle.

Create a function that takes a number step (which equals HALF the width of a square) and returns the amount of empty squares. The image shows the squares with step 1, 2 and 3. The return value is the number of cells not on a diagonal, which is 0 for the first square, 8 for the second, and 24 for the third.

Examples

empty_sq(1) ➞ 0

empty_sq(3) ➞ 24

empty_sq(10) ➞ 360

Notes

• Test input will always be a positive integer.
• The width of the square will always be even.