[2008] Finding Distance Between Unevenly Spaced Units?

I'm trying to write a little program to help me find the distance (in feet/inches) between the frames of a ship. This would be fairly easy, except the frames are unevenly spaced.Let's say this is a top view of a shape, and the frames are vertical lines that comprise the overall ship. (In ships, this is the shape of the hull.) Each number is a frame.

1 2 3 4 5 6 7 8 9 10 11 12 13

In this example, there are 1 units between frames 1-3, two units between frames 3-6, three units between 6-9, four units between 9-11, and 1 unit between 11-13.So, what I want to do is post a program where the user inputs two frame numbers: so they would enter, for instance, a start location of 5 and an end location of 11. So, it would calculate the number of units between those two frames based on the limits. So, between 3-6 there are two units, so it would subtract the start location from the nearest limit (6).

6-5 = 1

Multiply by 2 units spacing per frame, so that's 2 units. Frames 6-9 are all included, so 9-6=3 x 3 units = 9 units. Since 9-11 are also all included, it would be 11 - 9 = 2 x 4 units = 8 units. Then the program would add that all together - 2 + 9 + 8 = 19 units total.

Now, for my instance, there are a few more complications (it's in inches, and large increments (i.e. 36" spacing), and there are 304 total frames split between the middle of the ship in two sections, fore and aft of the line), but really I'm just trying to get a handle on how to impose these limits and calculate the distance. Attached is the proposed layout for my program (ignore the 20'5", that's just how I want the output to look).