Adding Items in a Grid

I created a grid in SAIL. I have managed to add and delete rows and capture the values. One of the columns in my grid calculates the difference in days between two dates. I need to have a field called Summary of Total Days contain the total of all those differences. I created an integer field on my interface but am having difficulties making that calculation. This is the code I have in my add a row rule to calculate the difference:

a!integerField(
label: "Total Days" & ri!index,
value: todate(ri!items[ri!index].endDate)-todate(ri!items[ri!index].startDate),
saveInto: ri!items[ri!index].amount,
validations: {}
),

Is my saveInto statement at fault. I am trying to bring back the value back into my interface that calls it. I have attached a screen shot of the interface that the customer would see.

OriginalPostID-271614