Srno in crystal report or Serial number in crystal report or srno in formula

numberVar f_sno=0;
stringVar f_booth="";
WhileReadingRecords;

if({command.booth_no} = f_booth) then
(
f_booth := {command.booth_no};
f_sno := f_sno + 1;
)
else
(
f_booth := {command.booth_no};
f_sno :=  1;
)