Custom Mathematical CAPTCHA in C# Asp.net

<form id="form1" runat="server">

    <div>

    <table cellpadding="2" cellspacing="0" border="1" style="width: 500px; border: dashed 2px #04AFEF;

            background-color: #B0E2F5">

        <tbody><tr>

            <td colspan="2">

                <h3>

                    Give your Details.</h3>

                <asp:ValidationSummary ForeColor="Red" runat="server" ValidationGroup="v" ID="ValidationSummary">

            </asp:ValidationSummary></td>

        </tr>

        <tr>

            <td >

                <b>Name</b>

            </td>

            <td >

                <asp:TextBox runat="server" id="txtName" width="60%"></asp:TextBox>

                <asp:RequiredFieldValidator id="RequiredFieldValidator1" runat="server" ErrorMessage="Please enter Your Name." ForeColor="Red" Display="Dynamic" ControlToValidate="txtName" ValidationGroup="v" >

            </asp:RequiredFieldValidator></td>

        </tr>

        <tr>

            <td>

                <b>Mobile</b>

            </td>

            <td>

                <asp:TextBox runat="server" width="60%" id="txtMobile" MaxLength="10"></asp:TextBox>

                <asp:RequiredFieldValidator id="RequiredFieldValidator2" ControlToValidate="txtMobile" runat="server" ForeColor="Red" ErrorMessage="Please Enter Your Mobile." Display="Dynamic" ValidationGroup="v" >

               </asp:RequiredFieldValidator><asp:RegularExpressionValidator ID="RegularExpressionValidator2"  ControlToValidate="txtMobile"

                                                    runat="server" ValidationGroup="v" ValidationExpression="(\d*-)?\d{10}"

                                                    Display="Dynamic"  ErrorMessage="Invalid Mobile No" ForeColor="Red"></asp:RegularExpressionValidator> </td>

        </tr>

        <tr>

            <td style="vertical-align: top;">

                <b>Description</b>

            </td>

            <td>

                <asp:TextBox runat="server" id="txtDescription" textmode="MultiLine" height="200px" width="100%"></asp:TextBox>

                 <asp:RequiredFieldValidator id="RequiredFieldValidator3" controltovalidate="txtDescription" ForeColor="Red" runat="server" ErrorMessage="Please Enter Your Description." Display="Dynamic" ValidationGroup="v" >

               </asp:RequiredFieldValidator></td>

        </tr>

        <tr>

            <td>

                <b>

                    <asp:label id="lblStopSpam" runat="server"></asp:label> = </b>

            </td>

            <td>

                <asp:TextBox runat="server" id="txtStopSpam"> </asp:TextBox>

                <asp:RequiredFieldValidator id="RequiredFieldValidator4" ForeColor="Red" ErrorMessage="Please enter your answer." Display="Dynamic" validationgroup="v" ControlToValidate="txtStopSpam" runat="server">

                </asp:RequiredFieldValidator><asp:CompareValidator id="CompareValidator1" ForeColor="Red" ErrorMessage="Invalid text format." Display="Dynamic" ValidationGroup="v" ControlToValidate="txtStopSpam" runat="server" Operator="DataTypeCheck" Type="Integer">

            </asp:CompareValidator></td>

        </tr>

        <tr>

            <td>

                 

            </td>

            <td>

                <asp:button id="btnSubmit" text="Submit" runat="server" validationgroup="v" onclick="btnSubmit_Click">

            </asp:button></td>

        </tr>

    </tbody></table>

    </div>

    </form>

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

        if (!Page.IsPostBack)

        {

            //===== Add text to stop spammer.

            generateText();

        }

    }

    void clear()

    {

        txtName.Text = "";

        txtMobile.Text = "";

        txtDescription.Text = "";

        txtStopSpam.Text = "";

    }

    protected void btnSubmit_Click(object sender, EventArgs e)

    {

        if (txtStopSpam.Text == ViewState["cal"].ToString())

        {

            //---- Do your operations here.

            //--- I have shown confirmation message.

            CustomValidator val = new CustomValidator();

            val.ValidationGroup = "v";

            val.IsValid = false;

            val.ErrorMessage = "Congratulations you have successfully solved the CAPTCHA.";

            this.Page.Validators.Add(val);

            //==== Create new spam protection code.

            generateText();

            clear();

        }

        else

        {

            CustomValidator val = new CustomValidator();

            val.ValidationGroup = "v";

            val.IsValid = false;

            val.CssClass = "valFailure";

            val.ErrorMessage = "You have entered invalid captcha code. Please retry.";

            this.Page.Validators.Add(val);

            //---- Generate new captcha code.

            generateText();

        }

    }

    private void generateText()

    {

        Random ran = new Random();

        //--- Here I have used numbers between 1 to 9 you can increase as per your req.

        int firstNumber = ran.Next(1, 100);

        int secondNumber = ran.Next(1, 100);

        ViewState["cal"] = firstNumber + secondNumber;

        lblStopSpam.Text = firstNumber.ToString() + " + " + secondNumber.ToString();

    }

}