Excel has a SUM button (on a toolbar). I can’t find a comparable button in LibreOffice.
It’s almost the same, a button on the Formula toolbar, with a Sigma (sum sign) image. If you don’t see it then check in menu View that Formula Bar is activated.
Go to view at the top ( in between the Edit and Insert Buttons ) then scroll down to Toolbars , then in the column on the right that comes up , scroll down and click on the Formula bar , then it will show up on the tool bar at the top .