using System;

class Deai
{
	static void Main(String[] args)
	{
		int px, py;
		int qx, qy;
		string[] vals = Console.ReadLine().Split(' ');
		px = int.Parse(vals[0]);
		py = int.Parse(vals[1]);
		vals = Console.ReadLine().Split(' ');
		qx = int.Parse(vals[0]);
		qy = int.Parse(vals[1]);

		int dx, dy;
		double t;
		dx = Math.Abs(px-qx);
		t = dx / 2.0;
		dy = Math.Abs(py-qy);
		t += dy / 2.0;
		Console.WriteLine(t);
			
	}
}