import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    double ax = sc.nextDouble();
    double ay = sc.nextDouble();
    double bx = sc.nextDouble();
    double by = sc.nextDouble();
    double ans = Math.abs(Math.max(ax,bx)-Math.min(ax,bx)) + Math.abs(Math.max(ay,by)-Math.min(ay,by));
    System.out.println(ans/2);
  }
}