I like to calculate in a table the difference between the value in a field in the current row and the previous field. How would I do that?

Something like:

|-------+------------|
| value | difference |
|-------+------------|
|    12 |            |
|    15 |          3 |
|    83 |         68 |
|    87 |          4 |
|    85 |         -2 |
|-------+------------|