add mileages together for the state

Trying to come up with a way to add a series of numbers together. I have Column A with a state in it, Column B has starting mileage in that state, Column C has ending mileage in that state, Column D is the difference (or total miles driven within the state). Need to figure out how to add all of the different state's miles driven together without clicking each state. I have a column of 679 values that I need to apply this too. Here is a small chunk for visual...Can you Help me?
State Start Mile End Mile Total Miles
MD 109680 109725 45
VA 109725 109830 105
VA 109830 109935 105
MD 109935 110050 115
MD 110050 110055 5 MD Total miles:
DE 110055 110070 15 NJ total miles:
NJ 110070 110220 150 VA Total Miles:
NY 110220 110355 135 DE total Miles:
NJ 110355 110505 150 NY total Miles:

Thank you so very much it works!!!