You can sum a series of cells very quickly by using the sum button, which is located in the toolbar.
To sum a range of cells, select the cell where you wish the result to be shown. Then press the Sum button in the toolbar. Office2 will try to intelligently select the range of cells that it thinks you wish to sum. If this selection is correct, press the Done button on the keyboard. If it is not correct, tap on the formula bar and manually change the cell range that you wish to sum and press the Done key.