C# Calculate the distance function between two points with known longitude and latitude

public Task<double> GetDistance(double lat1, double lon1, double lat2, double lon2)
        {
            int R = 6371;


            double rLat1 = ToRadian(lat1);
            double rLat2 = ToRadian(lat2);


            double dLat = rLat2 - rLat1;
            double dLon = ToRadian(lon2 - lon1);


            double a = Math.Pow(Math.Sin(dLat / 2), 2) +
                Math.Pow(Math.Sin(dLon / 2), 2) *
                Math.Cos(rLat1) * Math.Cos(rLat2);


            double b = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));


            return Task.FromResult((R * b));
        }