using System;

class S20_encounter{
  public static int Main(string[] args){
    double Ax, Ay, Bx, By;
    double time = 0;

    Ax = double.Parse(Console.ReadLine());
    Ay = double.Parse(Console.ReadLine());
    Bx = double.Parse(Console.ReadLine());
    By = double.Parse(Console.ReadLine());

    time += Math.Abs(((Ax - Bx) + (Ay - By))/2);

    Console.WriteLine("{0}",time);
    return 0;
  }
}