import java.util.Scanner;

public class B {
    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        double x1 = cin.nextDouble();
        double y1 = cin.nextDouble();
        double x2 = cin.nextDouble();
        double y2 = cin.nextDouble();
        System.out.println(String.format("%.3f", Math.hypot(x1 - x2, y1 - y2)));
        cin.close();
    }
}