MSSQL Executing Procedure Using IF on Scalar

I have a query that compares values in two columns, row by row, and sums the ABS(diff) to a single scalar int. I want to use the final scalar int to determine if a procedure should be executed.

The query has two WITH and a single SELECT FROM the 2nd WITH like this:

WITH AAA AS (...),    
BBB AS (... FROM AAA)    
SELECT SUM(I)    
FROM BBB

The query works - I get the correct result when it's run as SELECT.

However, when I try to use an IF [result] = 0, I get syntax errors no matter where I put the IF condition.

Is there any way to combine WITH and IF in the way I want?