Sum expression by variable with calculation

[Denzyl]

I am trying to create a Sum expression with a condition based on a variable minus n.  

I have a variable vLatestMonth which has the value of 7. 

I am trying to sum the [amount] column where the [month] column is equal to vLatestMonth minus one. 

I have tried the following code but received the following error message:

SUM({<[month]={$(vLatestMonth)-1}>}[amount])

Error in set modifier ad hoc element list: ',' or ')' expected

[Denzyl · Accepted solution]

I have found a solution:

SUM({<[month]={$(=vLatestMonth-1)}>}[amount])