Total sum of 18 individual numeric fields from single page

[Henry Figueroa]

I am trying to add up 18 individual fields with the same prefix name and a sequential number as a suffix, however, I cannot figure out a way to sum them all up yet in a function or a set of nested functions. I get an error stating that the system cannot calculate the 'formula'. My goal is to imitate the Sum() function from Excel. Any idea?

Field names used:  DaysTaken1 ... DaysTaken18

For each field, I am using the Start Date and End Date fields to calculate the days taken. For example, in the field formula, I use:

DaysTaken1 = ROUND((EndDate1 - StartDate1)/86400000, 1).

Sequentially, I have DaysTaken2 = ROUND((EndDate2 - StartDate2)/86400000, 1) until I reach to DaysTaken18.

The fields are in an IFTTT - onModified - Create Page. 

I need to add them all up into a single field for a report that I created. Please advise or if there is a better way to get the sum figure would be helpful. Many thanks, as always.