Keystrokes involved when using the "sum" forumula

[Silverthunder]

Excel Windows for Office 365

I click on a cell
1. type =sum(
2. I click on the first cell
3. I type :
4. I click on the last cell of that cells that I want to be summed up

When I click on the 2nd cell, Excel erases part of what I already have entered. For example "A1:" gets replaced with A20.

What's the most efficient keystrokes to use, given that you want to select the cells instead of typing the cell references?

[TMS]

What you describe should work.

Alternatively, you can select cells A1 down to A21 and click on the AutoSum icon on the ribbon.

[FlameRetired]

Also

After entering =sum(
click on first cell
while holding down the Shift key click the last cell