error: input string was not a correct format

Question

private void txtdiscountallowed_TextChanged(object sender, EventArgs e)

{

double discount = Convert.ToDouble(txtdiscountallowed.Text);

double sprice = Convert.ToDouble(txtSalesPrice.Text);

double Nudiscount = sprice * discount / 100;

txtCostPrice.Text = (sprice - Nudiscount).ToString();

}