I have been thinking of this but cant figure it out:

For example I have a Col start from row 10 (B10) going down. The number of row can be variable, but it should be odd number, the last row will not be count. Now I want the SUM of the subtraction of every 2 cell next together. Ex: I have B10:B21 then I need (B2-B1)+(B4-B3)+(B6-B5)+...+(B20-B19). How can I formular this?

Tkss!