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]);

        topX = ChangeFunc(topX);
        topY = ChangeFunc(topY);
        bottomX = ChangeFunc(bottomX);
        bottomY = ChangeFunc(bottomY);

        double xSum = topX + bottomX;
        double ySum = topY + bottomY;

        xSum /= 2;
        ySum /= 2;
        Console.WriteLine(xSum + ySum);
    }

    public static double ChangeFunc(double num)
    {
        if (num < 0)
        {
            num *= -1;
            return num;
        }
        return num;
    }
}