How do I format a double to a string and only show decimal digits when necessary?

I have code like:

lblFranshizShowInvwNoskhehEdit.Text = string.Format("{0:n}",
    (double)(int.Parse(drDarman["FranshizDarsad"].ToString()) * 
        Convert.ToInt64(RadNumerictxtPayInvwNoskhehEdit.Text)) / 100);

But {0:n0} string format forces the label's text to not have decimal digits and {0:n} string format forces the label's text to have 2 decimal digits (default).

In my scenario I just want decimal digits when necessary / without rounding them / how can I do that?