how do I take a two digit number, separate the two digits, add them together, show result

I am trying to create a small sheet that applies an algorithm to a nine digit barcode number to calculate the check digit which becomes the 10th.

Attached is the spreadsheet. With the logic of the algorithm included in it.

I've created the method of applying the multiplier to the individual digits of the barcode (In Row 6)

The result of each calculation is in Row 7.

The difficulty is that if the result is a number greater than 9, (in other words two digits) then the number needs to be split - e.g. 14 becomes 1+4 or 18 becomes 1+8, with the sum being the number displayed.

I hope that makes sense - the algorithm is explained in the spreadsheet attached.

I think something like IF result is < 9 , then subtract 10 and add 1 might do it....? But I don't know how to structure that in the cell.

Any help appreciated!

Greg