using System;

public class Program

{

    public static void Main()

    {

        double R = 6371; // km

        double Longitude1 = -51.139723;

        double Longitude2 = -51.120416;

        double Latitude1 = -23.330402;

        double Latitude2 = -23.336929;      

        double lat1 = Latitude1 * (Math.PI / 180.0);

        double lat2 = Latitude2 * (Math.PI / 180.0);

        double lon1 = Longitude1 * (Math.PI / 180.0);

        double lon2 = Longitude2 * (Math.PI / 180.0);

​

        double sLat1 = Math.Sin(lat1);

        double sLat2 = Math.Sin(lat2);

        double cLat1 = Math.Cos(lat1);

        double cLat2 = Math.Cos(lat2);

        double cLon = Math.Cos(lon1 - lon2);

​

        double cosD = sLat1*sLat2 + cLat1*cLat2*cLon;

​

        double d = Math.Acos(cosD);

​

        double dist = R * d;

​

        Console.WriteLine("{0}", dist);

    }

}