As you are creating/adding new list item/row for each month (For example: June & August), you will not be able to get the combined figure using SharePoint calculated column.
SharePoint calculated column formulas cannot read values from different list items/rows, it can read column values from same item/row only - This is by design limitation of SharePoint calculated columns.
You cannot reference a value in a row other than the current row.
Microsoft Documentation Source: Introduction to SharePoint formulas and functions
You may need to use another way for your requirements like using scheduled (daily, etc.) Power automate flow to read items from SharePoint list and get the combined figure OR customize the list forms using Power Apps and calculate the combined figure using Power Apps formula/functions while saving/updating list items.