Need help creating a new column using existing column

Hi guys,

I was wondering if anyone knew how to create a new variable by performing a simple mathematical operation on an existing column.

I've shown the operation im trying to do in the NEW COLUMN:

EXISTING_____NEW COLUMN
-1 ....................
-5 .................... (-5) - (-1)
7 ..................... ( 7) - (-5)
-3 .................... (-3) - ( 7)
2 ..................... etc...


I basically need to create a new column that subtracts existing observation #1 from #2, subtracts existing observation #2 from #3, and so on...

Any ideas as to how I can write a code for this?

Thanks in advance.