import java.util.Scanner;

 public class Circle extends Point{
	 	  
     public static void main(String[] args) {
    
	 Scanner sc = new Scanner(System.in);
	 Point c=new Point(); //Create a point center
	 c.x=sc.nextDouble();
	 c.y=sc.nextDouble();
	 Point p=new Point();  //Create a point on circumference
	 p.x=sc.nextDouble();
	 p.y=sc.nextDouble(); 
	 Circle c1=new Circle(); //Create an object of class Circle
	 c1.distance(c,p);  //Calcualte radius of the circle
	
  }
 
}

class Point{
  double x;
  double y;
    
  public static void distance(Point p1,Point p2){
        double d;
	  d=Math.sqrt((p2.x-p1.x)*(p2.x-p1.x) + (p2.y-p1.y)*(p2.y-p1.y));
	  System.out.println(d);
  }	 
}