Counting field values

Yes, there should be 8 number fileds on the form, but I would like to have another one - N/A (when data is not provided). If data are provided for only some age ranges, I should assume the rest are zero. N/A only to be used if no data provided at all (so should be left blank the field of totals aswell). Yes, I think that would be the best - that Access checks whether the totals column matches with sum after all fields antered. If they don't match I would like to have the message popping up that the sum of hose 8 fields should be equal to totals. Perhaps there is a way to set up each field that it is not greater than total field. .....