while executing this code below I got this error "Input string was not in a correct format." and it highlight "double amt = Convert.ToDouble(txtAmt.Text);".how can I correct it?

DateTime dt1 = Calendar2.SelectedDate;
            DateTime dt2 = Calendar1.SelectedDate;
            TimeSpan ts = dt1 - dt2;           
            double days = ts.Days;
            txtDays.Text = days.ToString();
            double res = days - 1;
            double amt = Convert.ToDouble(txtAmt.Text);
            double res1 = amt * res;
            double amtperday = Convert.ToDouble(txtAmtPerDay.Text);
            double res2 = days * amtperday;
            double tot = res1 + res2;
            txtTotAmt.Text = tot.ToString();