On the status bar, look at the value next to Sum. Type a few numbers in a column, or in a row, and then select the range of cells that you just filled.
If you see a date instead of the result that you expected, select the cell, and then on the Home tab, select General. If you change the value of C1 or D1 and then press RETURN, the value of E1 will change, even though the formula did not.
