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

class Program {
    static string ReadLine() { return Console.ReadLine(); }
    static int ReadInt() { return int.Parse(ReadLine()); }
    static int[] ReadInts() { return ReadLine().Split().Select(int.Parse).ToArray(); }
    static string[] ReadStrings() { return ReadLine().Split(); }

    static void Main() {
        var p = ReadInts();
        var q = ReadInts();

        var dx = Math.Abs(p[0] - q[0]);
        var dy = Math.Abs(p[1] - q[1]);

        double ans = dx/2 + dy/2;
        int n = (dx % 2) + (dy % 2);
        if (n == 2) {
            ans++;
        }
        else if (n == 1) {
            ans += 0.5;
        }
        Console.WriteLine(ans);
    }
}