you are most likely putting the formula sgttim gave you in the select expert. That formula area is only for creating a boolean (true/false) evaluation to determine if each row should be included or excluded from the report.
In the field explorer there is a an option for "Formula Fields".
Right click on that and select new.
That is where you create new formulas to be used in the report.
That being said, if your data is more dynamic I think I would use a crosstab to get what you want.
Create a formula (as above) to combine F1 and F2
{table.F1} & {table.F2}
insert a crosstab in the report header or footer
insert the above formula field as teh Row
insert F3 as a sum in the summarized fields
play around with the formatting to make it visually what you want
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