Formula for Calculating Cumulative Mileage

I need help with a formula... Attached is my work.

I have a spreadsheet that tracks mileage for our fleet. At the end of every month, we get odometer readings from our vehicles. The thing is, sometimes one or more of our vehicles are in maintenance for the month and does not get driven, so there's a blank cell, and it throws off the formula, so it has to be manually calculated. 

 

 

I tried using this formula in row 2 =(C2-B2)+(D2-C2)+(E2-D2)+(F2-E2)+(G2-F2)+(H2-G2)+(I2-H2)+(J2-I2)+(K2-J2)+(L2-K2)+(M2-L2)+(N2-M2)

 

I should be getting 341, however, since the vehicle can be in maintenance, or the year is not finished, I get -47819. 

 

Is there a better formula that can account for the months where the vehicle is in maintenance and no mileage is gained but still acquire the total mileage for the year? 

Any help would be much appreciated!