Im trying to make a column with the following variable (23 - x - y - z).

23 is a base, then I want to subtract the entered numeric values in cells x, y, z (where xyz have been defined by the field name)

In the formula field I entered

(23 - Sep_2009 - Oct_2009 - Nov_2009 - Dec_2009)

But keep getting an error

Error in statement at line number 0
The Left expression is of type STRING and the right expression is of type BIGINT and the operator - is not valid here

Any ideas? Thanks!