Hi,
DBlank asked for me to help you out with shared variables as this is a method to accomplish what you are looking at.
Shared variables usually involve 3 formulas, 1) to initialize the variable (usually 0, but not in this case), 2) increment the variable, and 3) display the results. In your case they would look something like:
in the group header, initialize:
shared numbervar counter := 0; //on second thought, this might be better
"" //to hide the formua's output, you could supress the formula (I think)
in the detail section:
shared numbervar counter;
if counter = 0 then
counter := {table.field}
else
counter := counter - {table.field};
"" //again to hide the output (remove this line for debugging)
in the group footer:
shared numbervar counter
this can also be used for the differencing if desired with but a few modifications, but if something is working, why mess with it ;)
HTH