using System;

class Entrypiont
{
    static void Main()
    {
        string[] Input = Console.ReadLine().Split(' ');
        int Sickness = Convert.ToInt32(Input[0]);
        int Pereplet = Convert.ToInt32(Input[1]);
        int Start = Convert.ToInt32(Input[2]);
        int Finish = Convert.ToInt32(Input[3]);

        int totalSickness = Sickness + Pereplet * 2;

        int way = 0;
        if ((Finish - Start) > 0)
        {
            int bookCount = Math.Abs(Finish - Start) - 1;
            way = totalSickness * bookCount + 2*Pereplet;
        }
        else if ((Finish - Start) < 0)
        {
            int bookCount = Math.Abs(Finish - Start) - 1;
            way = totalSickness * bookCount + 2 * (Pereplet + Sickness);
        }
        else
        {
            way = Sickness;
        }

        Console.Write(way);
    }
}

it's very bad to post here AC solutions ;D

...especially when they are far far away from ideal =)

thanks, because I'm not understand this problem/// :)