getting error when i try this code

Question

             double days = Convert.ToDouble(txt_working.Text);
             double sundays = Convert.ToDouble(txt_netpay.Text);
             double a = Convert.ToDouble(txt_basicscale.Text);
             txt_basicamount.Text = Convert.ToDouble((a / days) * sundays).ToString();


when i try this above code i got error as

    in second line input sting was not in a correct format.


please help me