using System;

public class Program

{

    public static void Main()

    {

        double lat1 = 10.178590;

        double lat2 = 10.156445;

        double lon1 = 123.500404;

        double lon2 = 123.534304;

​

        var R = 6378.137; // Radius of earth in KM

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

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

        var a =

            Math.Sin(dLat / 2) * Math.Sin(dLat / 2) +

            Math.Cos(lat1 * Math.PI / 180) * Math.Cos(lat2 * Math.PI / 180) *

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

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

        string x = (R * c).ToString();

        var d = (R * c);

        Console.WriteLine(d);

    }

}