I am having three text boxes as below

Question

 i am having three text boxes as below

   Rent           textbox

   service tax   textbox

   Gross rent    textbox


i am adding rent and serice tax output will be displayed in gross rent.

i write caluation code in txt_tax as below

private void txt_tax_TextChanged(object sender, EventArgs e)

        {

            int a, b, c;

            a = Convert.ToInt32(txt_rent.Text);

            b = Convert.ToInt32(txt_tax.Text);

            c = a + b;

            txt_grossrent.Text = c.ToString();

        }

for example i type service tax amount as 1000 then i clear the 1000 amount

the error occurs as   Input string was not in a correct format.

please help me.please give the correct code.

Regards 

Narsiman