How to get payment id from atom payment gateway integration response page

protected void Page_Load(object sender, EventArgs e)
{
try
{
    if (!IsPostBack)
    {
    NameValueCollection nvc = Request.Form;
    string mmp_Transaction = nvc["mmp_txn"].ToString();
    string PaymentStatus = nvc["f_code"].ToString();
    string MerchantTransactionId = Request.Form["mer_txn"];

    string Amount = Request.Form["amt"];
    string Product = Request.Form["prod"];
    string TransactionDate = Request.Form["date"];
    string BankTransactionId = Request.Form["bank_txn"];
    string ClientCode = Request.Form["clientcode"]; // Encrypted User Information
    string BankName = Request.Form["bank_name"];

    //User Details
    string Udf1 = Request.Form["udf1"]; //First Name
    string Udf2 = Request.Form["udf2"]; //Email
    string Udf3 = Request.Form["udf3"]; //Mobile
    string Udf4 = Request.Form["udf4"]; //Address
    string Udf5 = Request.Form["udf5"]; //Bank Name
    string Udf6 = Request.Form["udf6"]; //EMI Option

    string strResponceIP = HttpContext.Current.Request.UserHostAddress;
    string strRemarks = "txnId:" + MerchantTransactionId + ", txnStatus:" + PaymentStatus + ", amount:" + Amount + ", pgTxnId:" + mmp_Transaction + ", BankTransactionId:" + BankTransactionId + ", Udf1:" + Udf1 + ", Udf2:" + Udf2 + ", Udf3:" + Udf3 + ", Udf4:" + Udf4 + ", Udf5:" + Udf5 + ", Udf6:" + Udf6;


    string ATOMPaymentIP = ConfigurationManager.AppSettings["ATOMPaymentIP"].ToString();//203.114.240.183--Stage
    //Verify the Response Server
    if (Request.UrlReferrer.Host.Trim() != ATOMPaymentIP)
    {
        // REDIRECT TO ERROR PAGE IF RESPONSE IP IS NOT VERIFIED
    }

    // CHECK THE PAYMENT STATUS AND VALIDATE THE TRANSACTION ID
    if (PaymentStatus.ToUpper().Trim() == "OK" && transactionid.Trim()==MerchantTransactionId.Trim())
    {
        //"Transaction proceed successfully."

        // UPDATE THE TRANSACTION STATUS IN DB
        //
        // CODE FOR UPDATE STATUS
        //
        //

        // REDIRECT TO SUCCESS PAGE OR SHOW THE PAYMENT RESPONSE ON SAME PAGE

    }
    else
    {
        // PAYMENT STATUS IS NOT SUCCESS OR TRANSACTION ID IS NOT VERIFIED
        // REDIRECT TO ERROR PAGE
    }
    }
}
catch (Exception ex)
{
    // HANDLE ERROR
}
}