Averaging

I'm trying to average (=average(f7:f40) the differance between two values. This is the formula (=c6-c7, =c7-c8, and so forth) I use to calculate the differance. Sometimes the value is a negative number witch throws the average off for the differance. I need all the values calculated to be a positive number. How do I write a formulae that will turn a negative result into a positive?