Sum of the field of formula column

Hi,

I have created formula filed "Amount" as follows:

amount : amt1+amt2+amt3;

Now I want to sum up the formua filed "Amount" to show it on report.

I have tried creating another formula filed "Totals"

totals := Sum({Amount});

Here i am getting an error like filed cannot be summed.

what is wrong in it? How can I summarize my formula field.