import java.util.Scanner;
import java.lang.Math;
public class succ {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double x1, x2, y1, y2, a, b, c;
        x1 = sc.nextDouble();
        x2 = sc.nextDouble();
        y1 = sc.nextDouble();
        y2 = sc.nextDouble();
        a = (x2-x1)*(Math.cos(x1*(Math.PI/180)));
        b = (y2-y1);
        c = Math.sqrt(a*a+b*b)*((Math.PI*12756.274)/360);
        System.out.println(c);
    }
}