using System;

public class Program

{

    public static void Main()

    {

        var lat1 = 55.774530;

        var lon1 = 12.519670;

        var lat2 = 55.770291;

        var lon2 =  12.503841;

        var R = 6371e3; // metres

        var φ1 = lat1 * Math.PI/180; // φ, λ in radians

        var φ2 = lat2 * Math.PI/180;

        var Δφ = (lat2-lat1) * Math.PI/180;

        var Δλ = (lon2-lon1) * Math.PI/180;

​

        var a = Math.Sin(Δφ/2) * Math.Sin(Δφ/2) +

                  Math.Cos(φ1) * Math.Cos(φ2) *

                  Math.Sin(Δλ/2) * Math.Sin(Δλ/2);

        var c = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1-a));

​

        var d = R * c; // in metre

        Console.WriteLine(d / 1000);

        Console.WriteLine(R);

    }

}