Please post what's in your formula and the version of Crystal.
You might use a manual summary against it, so use the 3 formula method, you posted nothing technical so I can only guess):
Report Header Formula:
whileprintingrecords;
Numbervar MyTotalValue:=0;
Detail formula:
whileprintingrecords;
Numbervar MyTotalValue;
if {table.field} = "C"
and previous(Table.field} = "C" then
MyTotalValue:={table.value}+Previous({table.value});
Report Footer Formula:
whileprintingrecords;
Numbervar MyTotalValue
This should get you by, you might need to modify it a bit for groupings, etc.
-k