import java.util.Scanner;
public class CoordinateGeometry {
public static void main(String [] args) {
Scanner scnr = new Scanner(System.in);
double x1;
double y1;
double x2;
double y2;
double pointsDistance;
double xDist;
double yDist;
pointsDistance = 0.0;
xDist = 0.0;
yDist = 0.0;
x1 = scnr.nextDouble();
y1 = scnr.nextDouble();
x2 = scnr.nextDouble();
y2 = scnr.nextDouble();
poinsDistance = Math.sqrt(Math.pow(x2 - x1, 2) + (Math.pow(y2 - y1, 2));
System.out.println(pointsDistance);
}
}