update a value at the current index then increment at other indexes

Hi, In foreach loop I am fetching some value. So the requirement is at current index of foreach loop I am updating some default value and then at the proceeding indexes i want to increment by one in the default value. Here is how I am doing it.

a!forEach(
items: local!rawData_at,
expression:
if(fv!index = 1,

a!update(fv!item,"KeyTaskModel",ri!KeyTaskModel_StartingOrder_int),
a!update(fv!item,"KeyTaskModel",fv!index+1)

))

What it is doing is i have saved the value from rule input ri!KeyTaskModel_StartingOrder_int at the first index but the subsequent indexes is adding value to the index value not the rule input value. How can I achieve that? attaching the output below.