Custom Spreadsheet Add/Subtract Functions

I have an existing spreadsheet that needs two new integer-based functions—one for addition, one for subtraction. Each must accept multiple cell references, perform the calculation across every integer found, and drop the single result into a chosen cell of the same sheet.

I will call these routines manually through a specific command, so please expose each as a clear, callable macro or custom function name. No automatic triggers or on-change events are required.

All I need back is the working code embedded in the file (VBA, Google Apps Script, or another native solution that integrates cleanly with a standard spreadsheet), plus a brief note that shows the exact syntax I’ll type to run each command.