Why?
When you create a formula, Excel does your math for you.
How?

Click the cell where you want the result to appear (for example, B5).

Click in the formula bar, and then type an equal sign (=).
The formula appears in the cell and in the Formula Bar. If you don't see the Formula Bar, on the View menu, click Formula Bar.

Click the first cell that you want to include in the total (for example, B2), and then type a plus sign (+).

Click additional cells to capture all the data that you want to include in the total (for example, =B2+B3+B4), and then press RETURN.
The formula result appears.
Hints

Do not format numbers as you enter them in formulas. For example, even if the value that you want to enter is $1,000, enter 1000 in the formula.

Make sure that you match all open and close parentheses.