Subtracting and Inserting new field

Hi,

I have a table:

Metric          MetricDescription                       Season    Measure

01                       Cancellation                      Ho11         3

03                           GF                                HO11          4

I need to make a new Row Based on Same season. I need to subtract GF- Cancellation  and call it NS with 46 metric. So my table should look like

Metric          MetricDescription                       Season    Measure

01                       Cancellation                      Ho11         3

03                           GF                                HO11          4

46                          NS                                 HO11          1

Any Suggestion?

SPPandey