import java.util.Scanner;

class S20_encounter{
  public static void main(String[] args){
    double ax, ay, bx, by;
    double time = 0;
    Scanner scn = new Scanner(System.in);

    ax = scn.nextDouble();
    ay = scn.nextDouble();
    bx = scn.nextDouble();
    by = scn.nextDouble();

    time += Math.abs(((ax - bx)+(ay - by))/2);

    System.out.println(time);
  }
}