Adding TextBox value to decimal variable is appended in ASP.Net

i am calling sum from table into label below is code c#

I have one label lbduty.Text and second textbox is txttexamount.Text

if have have 5000 in lbduty.Text which called from table then i add 777 from textbox then it display (7775000) means that values is not getting sum into lbduty.Text

i used below for sum 

            Decimal a, b, c;
            //txttamt.Text = (Decimal.TryParse(txtrate.Text,out a ) * Decimal.TryParse(txtCno.Text,out b));
            bool isAValid = Decimal.TryParse(lblimit.Text, out a);
            bool isBValid = Decimal.TryParse(lbduty.Text, out b);
            bool isCValid = Decimal.TryParse(txttexamount.Text, out c);
            if (isAValid && isBValid && isCValid)
            {
                lbduty.Text = ((txttexamount.Text) + b).ToString();               
            }
            else
            {
                lbremain.Text = "Invalid input";
            }