An excellent number n has an even number of digits and, if you split the number into the front half a and the back half b, then b^2 − a^2 = n.
For example, 3468 = 68^2 − 34^2.
The only two-digit excellent number is 48 and the only four-digit
excellent number is 3468.

There are eight six-digit excellent numbers.

List them.

Bonus: List all 10-digit excellent numbers.

Source: Project Euler.