first I would suggest to not use a Classical Report for doing this, instead use the Interactive Grid.
Your query could look similar to
select ..., Full_casual_leave_cnt, Half_casual_leave_cnt, (Full_casual_leave_cnt+Half_casual_leave_cnt) as calc_column
Define the calc_column as Query only = "Yes"
Then you can declare two Dynamic Actions on the two Full_casual_leave_cnt and Half_casual_leave_cnt
Action: "Execute PL/SQL Code"
Code: :calc_column := .Full_casual_leave_cnt + :Half_casual_leave_cnt (use the : for binding the column values)
Items to Submit: Full_casual_leave_cnt, Half_casual_leave_cnt
Items to Return: calc_column
Hint: When you are expecting NULL values you should work with NVL(column,0) expressions