How to do it...

Variables are created by using the VAR statement with the following syntax:

VAR <name> = <expression>

 <name> is the name of the variable. Variable names must begin with the letters a-z or A-Z. The only other supported characters within variable names are the characters 0-9. Existing table names and certain keywords are not permitted as variable names.

DAX calculations that use a VAR statement must also use a RETURN statement.

By following these principles, we can write a DAX measure as follows:

Variable DAX = 
/*
 * This measure summarizes the table, Table, grouping by [Column] and summing [Column1]. This  
 * summarized table is then filtered to values 2 and 3 in [Column] and then sums up [Column1]
 *
 * Gregory J. Deckler
 * gdeckler@fusionalliance.com
 * 10/7/2019
 */
VAR __summarizedTable =                 // Summarize table by [Column], summing [Value]
    SUMMARIZE(
        'R03_Table',
        'R03_Table'[Column],
        "__Value",
        SUM( 'R03_Table'[Value] )
    )
VAR __filteredTable =                   // Filter summarized table for 2 and 3
    FILTER(
         __summarizedTable,             // Here we use our __summarizedTable variable
        [Column] = 2
        ||
        [Column] = 3
    )
VAR __sum =                             // Sum [__Value]
    SUMX(
        __filteredTable,                // Here we use our __filteredTable variable
        [__Value]
    )
RETURN                                  // If result is < 400, return the sum, otherwise -1
    IF(
        __sum < 400,                    // We avoid having to do the same calculation twice
        __sum,
        -1
    )