Hi Nimish,
If this sum is in the report footer (grand total) then create a formula with this code:
Sum({field to sum}) - 0.5
Drag and drop this formula on the report footer.
It this needs to be on the group footer, then the formula would be:
Sum({field to sum}, {group field}) - 0.5
Replace {group field} with the field used to group the report.
-Abhilash