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

class AB {

    static void Main() {
        int[] q = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        int []P= Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        Console.WriteLine(Math.Abs((float)(Math.Abs(P[0] - q[0]) + Math.Abs(P[1] - q[1]))) / 2);
    }
}