Measure to Calculate difference between values in existing column row by row

[RJ_KON]

Hi all, i am new to power BI and DAX. i got an issue in writing simple measure. Please see below my query.

Below is the table i am working on.

Period   Index     TC count     VARIANCE

P01           1             10402           Blank

P01           2            8925            (10402-8925)

P01           3              10258           (8925-10258)

P01           4            12216           (10258-12216)

My table name is VARIANCE_19_20

BELOW IS MEASURE I AM TRYING, BUT DIDN'T WORK.

VARIANCE = 
VAR NextIndex = VARIANCE_19_20[INDEX] - 1
RETURN
    VARIANCE_19_20[TC Count]
        - CALCULATE (
            VALUES ( VARIANCE_19_20[TC Count] ),
            FILTER ( ALL ( VARIANCE_19_20), VARIANCE_19_20[INDEX] = NextIndex)
        )

I am trying to write a measure to see difference in Variance column as shown in above table.

Please help