public class GreatCircle {
    public static void main(String[] args) {
        double x1 = Double.parseDouble(args[0]);
        double y1 = Double.parseDouble(args[1]);
        double x2 = Double.parseDouble(args[2]);
        double y2 = Double.parseDouble(args[3]);

        double r = 6371.0; // Earth radius

        double dx = Math.toRadians(x2 - x1) / 2.0;
        double dy = Math.toRadians(y2 - y1) / 2.0;

        double under = Math.sin(dx) * Math.sin(dx) +
                Math.cos(Math.toRadians(x1)) * Math.cos(Math.toRadians(x2))
                        * Math.sin(dy) * Math.sin(dy);

        double distance = 2.0 * r * Math.asin(Math.sqrt(under));

        System.out.println(distance + " kilometers");
    }
}