using System;

public class Program {
	public void Solve() {
		string[] A = Console.ReadLine().Split(' ');
		string[] B = Console.ReadLine().Split(' ');
		float x = Math.Abs(int.Parse(A[0]) - int.Parse(B[0]));
		float y = Math.Abs(int.Parse(A[1]) - int.Parse(B[1]));
		float dist = 2f;
		float time = x / dist + y / dist;
		Console.WriteLine(time);
	}
	
	public static void Main(string[] args) {
		Program program = new Program();
		program.Solve();
	}
}