SUM is the spreadsheet equivalent of an open invite: it welcomes everything into the total, whether it belongs there or not.

Alternatively, since the cells are consecutive, you could also use the SUM feature to sum multiple columns in Excel, based on criteria. In this instance, you would place the cursor in the C10 cell ...

I keep running into situations in Excel where I want to calculate a conditional sum, but my criterion is something along the lines of "Include cell B1 if cell A1 is a number, likewise B2 and A2 ...

You can sum a series of numbers in Microsoft Excel manually by typing in a simple formula, or automatically with a tool.