SUM VALUES BASED ON IIF CONDITION in RDLC REPORT

HI,

I am trying to SUM values form pay column based on nature column in RDLC report

the database table are like:

    Nature                       pay

(nvarchar(50))         decimal(18, 2)

PERMANENT                  1000.00

PERMANENT                  1000.00

ADHOC                          500.00

ADHOC                         500.00

I Just want to SUM the values based on nature column

Like if i want the total of PERMANENT then it must show 2000.00

and it i want the total of ADHOC the it must show 1000.00

Firstly i tried this EXPRESSION

=IIF(First(Fields!nature.Value, "DataSet1") = "PERMANENT","",SUM(CINT(Fields!pay.Value)))

But it Summing up all the values.

Then i also tried this EXPRESSION

=Sum(CINT(IIF(Fields!nature.Value = "PERMANENT","",Fields!pay.Value)))

But its giving -    #Error

Any Expert can direct me in the right direction...............?