How to Determine Averages of Increments

[MrDeath]

Alright, here's the situation. I have 24 months of sales information in which most of the time the month in question increased sales over the month prior. So 24 numbers in a line with most numbers higher than the one before it.

I need to find the average monthly incrimental value for those 24 months.

What would be a faster way to do it through excel than manually find the difference for each of the 23 pairs of months, add those up, and divide by 23? Is there any way to make a function or a couple of functions to speed up the process?