suppression - Crystal reports Sum if a previous field is the same to current field on section 3(details) -

hi im newbie programmer , have problem in crystal reports.

i have table named "payroll" , has fields(id,fullname,netsalary) have inserted 3 records on mysql table:

id|fullname|netsalary 1 |cris tiu|500 2 |mat joe |100 3 |mat joe |400 

how can make this? dont want group them fullname , give total instead if full name duplicated, display once , total net salary.

id|fullname|netsalary 1 |cris tiu|500 2 |mat joe |500 

i have tried adding formula contained code:

if {fulname}=previous({fullname}) sum({netsalary}) else {netsalary} 

but gives me display this:

id|fullname|netsalary 1 |cris tiu|"blank" 2 |mat joe |100 3 |mat joe |500 

please me work depends on this. thank in advance