using System;
using System.Collections.Generic;
using System.Linq;

public class Yuki410
{
	public static void Main()
	{
		var pxy = Console.ReadLine().Split().Select(x => int.Parse(x)).ToList();
		var qxy = Console.ReadLine().Split().Select(x => int.Parse(x)).ToList();
		
		var l = (double)(Math.Abs(qxy[0] - pxy[0]) + Math.Abs(qxy[1] - pxy[1]));
		Console.WriteLine(l / 2);
	}
}