Going further, as a general "rule of thumb," even after you master SUM, if you find yourself thinking "There ought to be a way to do _____________" you can take it for granted, first, that others have thought the same AND, second, that Excel probably DOES do it. Excel is amazingly powerful, and well worth taking the time to learn (a) some basics, but more than that, (b) how to navigate to features yet unknown.
The trick, admittedly, is to find those things you wish it could do, since terminology can vary. In a case like this, you could easily have Googled "how to do sums in Excel" or "how to add numbers in Excel" and I'm confident you'd find more than enough answers. Note the screen capture below; I didn't even have to finish the question for Google to offer quite a few answers, the first of which pointed to Excel!
I have used Excel for years, decades actually, and still am discovering features and capabilities. Here's one (of many) resources that I've found helpful. This one is on-line; there are also books by the hundreds. https://exceljet.net
