Remove the odd numbers for they can always be expressed as difference of squares are you have,
Divide them by 4 and find the one which leaves a remainder of 2.
Since both are even they leave either no remiander or 2.
Thus, find the number not divisible by 4.
The trick is to look at the last two digits,
66----> Not Divisible
68----> Yes Divisible
Thus, the answer is,