C# : Decimal points lost when adding two decimal numbers

99.99*33/100=32.99

then i need to add 33+99.99=132 i need answer but it shows 101

  double  s, su = 0;
        s = (double.Parse(TextBox17.Text) + Convert.ToInt32(TextBox13.Text)) / 100;
            //float.Parse(TextBox13.Text)) / 100;
        su = float.Parse(TextBox17.Text) + s;
        TextBox21.Text = Convert.ToString(su);