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();
    if(ax == bx && ay == by)System.out.println("0");
    else if(ax == bx)System.out.println((Math.abs(ay)+Math.abs(by))/2);
    else if(ay == by)System.out.println((Math.abs(ax)+Math.abs(bx))/2);
    else System.out.println((Math.abs(ax)+Math.abs(ay)+Math.abs(bx)+Math.abs(by))/2);
  }
}