class Program
{
    static void Main(string[] args)
    {
        string[] topNum = Console.ReadLine()!.Split(' ');
        string[] bottomNum = Console.ReadLine()!.Split(' ');
        double topX = double.Parse(topNum[0]);
        double topY = double.Parse(topNum[1]);
        double bottomX = double.Parse(bottomNum[0]);
        double bottomY = double.Parse(bottomNum[1]);
        
        double xSum = CalcFunc(topX, bottomX);
        double ySum = CalcFunc(topY, bottomY);

        xSum /= 2;
        ySum /= 2;

        Console.WriteLine(xSum + ySum);
    }

    public static double CalcFunc(double top, double bottom)
    {
        if (top < bottom)
        {
            double tmp = top;
            top = bottom;
            bottom = tmp;
        }

        return top - bottom;
    }
}