How to calculate difference between 2 columns excluding nulls

Here is my input table:

   TUE         MON        
----------- -----------
          2           -
          -          25
         27           -
          -          48
         50           -
          -          78
          3           -
          -          40
         42           -
          -          62
         65           -
          -          85
          4           -
          -          40
         42           -
          -          62
         68           -
          -          92
What is the best way to calculate difference between not NULL MON and
previous not NULL TUE 

25 - 2
48 - 27
........
62 - 40
92 - 68

I did it by creating separated tables for MON and TUE AND THEN JOIN BASED
ON
ROW NUMBER
Any more creative solutions.
Thank's in advance
Leny G.

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