I'm trying to write sum function in relational algebra with only basic operators without using aggregation function, but I don't know how or if it's possible or not!? I've searched but can't find anything on the internet

For example we have table order:

Code:
OrderID ProductID  Quantity
----------------------------
1001       15         5
1002       35         7
1002       10         10
1003       50         30
1004       47         15
We can sum up the quantity with sum function in sql

Code:
SELECT SUM(Quantity) FROM OrderTable
Or

Code:
Fsum(Quantity)(OrderTable)
How can I get the total quantity without using any aggregate functions?