using System;

namespace No306
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			var f = Console.ReadLine ().Split(' ');
			var s = Console.ReadLine ().Split(' ');
			var fx = Convert.ToDouble (f [0]);
			var fy = Convert.ToDouble (f [1]);
			var sx = - Convert.ToDouble (s [0]);
			var sy = Convert.ToDouble (s [1]);

			var dx = fx - sx;
			if (dx == 0) {
				Console.WriteLine (fy.ToString ());
				return;
			}
			var leftx = sx;
			var lefty = sy;
			var rightx = fx;
			var righty = fy;
			var midx = (leftx + rightx) / 2d;
			var midy = (lefty + righty) / 2d;
			while (Math.Abs(midx) > 1e-16) {
				midx = (leftx + rightx) / 2d;
				midy = (lefty + righty) / 2d;
				if (midx < 0) {
					leftx = midx;
					lefty = midy;
				} else if (midx > 0) {
					rightx = midx;
					righty = midy;
				}
			}
			Console.WriteLine (midy.ToString ());					
		}
	}
}