i've tried this codes but it doesn't work..

select sum((end_date-start_date+1)*(basic_sal/20))
into :xtra.unpaid_leave
from leave_appl,emp_mast
where emp_mast.emp_id=leave_appl.emp_id and
emp_mast.emp_id=:global.emp_id and
status='approved' and
type='unpaid'and
la_date like '%":xtra.mont"%';

notes:
:xtra.month is the list item where i'll pass the value