Answer: Start Volume Number > Stop Volume Number
You assume that worm start at lower volume and stops at higher volume but DO NOT.

Here is a code in C#:

using System;
class Program
{
    static void Main()
    {
        string[] ss = Console.ReadLine().Split(new char[] { ' ', '\t', '\n', '\r' }, StringSplitOptions.RemoveEmptyEntries);
        int Thickness = int.Parse(ss[0]);
        int CoverThickness = int.Parse(ss[1]);
        int Start = int.Parse(ss[2]);
        int Finish = int.Parse(ss[3]);
        int route = Math.Abs((Finish - Start - 1) * (Thickness + 2 * CoverThickness) + 2 * CoverThickness);
        Console.WriteLine(route);
    }
}