import java.lang.*;
import java.io.*;

class Yuki410{
  public static void main(String[] args)
  throws IOException {
    DataInputStream d=new DataInputStream(System.in);
    String s=d.readLine();
    String[] sa=s.split(" ");
    int x1,y1;
    x1=Integer.parseInt(sa[0]);
    y1=Integer.parseInt(sa[1]);
    String s2=d.readLine();
    String[] sa2=s2.split(" ");
    int x2,y2;
    x2=Integer.parseInt(sa2[0]);
    y2=Integer.parseInt(sa2[1]);
    int xa,ya;
    double za;
    xa=x1-x2;
    if(xa<0) xa*=-1;
    ya=y1-y2;
    if(ya<0) ya*=-1;
    za=xa+ya;
    za/=2;
    System.out.println(za);
  }
}