*pseudo code

If both positive:

ABS(a-b) or MAX(a,b) - MIN(a,b)

If both negative:

ABS(ABS(a) - ABS(b)) or MAX(a,b) - MIN(a,b)

If one positive and one negative:

MAX(a,b) - MIN(a,b)

Hence for all situations:

MAX(a,b) - MIN(a,b)