public class GreatCircle 
{
	public static void main(String [] args){


		double x1 = Math.toRadians(Double.parseDouble(args[0]));
		double y1 = Math.toRadians(Double.parseDouble(args[1]));
		double x2 = Math.toRadians(Double.parseDouble(args[2]));
		double y2 = Math.toRadians(Double.parseDouble(args[3]));
		
		double t1 = (x2 - x1) / 2.0;
		double t2 = (y2 - y1) / 2.0;
		double q1 = Math.pow(Math.sin(t1), 2);
		double q2 = Math.pow(Math.sin(t2), 2);

		double as = q1 + (Math.cos(x1) * Math.cos(x2) *  q2);
		double sq = Math.sqrt(as);

		double distance = 2 * 6371.0 * Math.asin(sq);

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