/* all the routines to calculate the distance */
/* between two points on the earth */

#include <math.h>

double pi=3.1415;

double deg_2_rad (double deg) {
  return deg*pi/180.0;
}

double rad_2_deg (double rad) {
  return rad*180.0/pi;
}

double geo_colat (double lat) {
  double r_lat;

  r_lat=.993277 * tan(deg_2_rad(lat));

  return (pi/2-atan(r_lat));
}

double calc_dist (double s_lat,double s_lon,double e_lat,double e_lon) {

  double gs_lat=geo_colat(s_lat);
  double rs_lon=deg_2_rad(s_lon);
  double ge_lat=geo_colat(e_lat);
  double re_lon=deg_2_rad(e_lon);

  double ans=rad_2_deg(acos(cos(ge_lat)*cos(gs_lat)+sin(ge_lat)*sin(gs_lat)*cos(rs_lon-re_lon)));

  return ans;

}