How to delete a row from GridView using ID from TextBox in ASP.Net

Please refer this code

HTML

<span>Roll No</span> &nbsp;
<asp:TextBox ID="txtRollNumber" runat="server" />
&nbsp;
<asp:Button ID="btnDeleteStudent" Text="Delete" OnClick="DeleteStudent" runat="server" />
<hr />
<asp:GridView ID="gvStudents" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
    runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField DataField="RollNumber" HeaderText="Roll Number" ItemStyle-Width="150" />
        <asp:BoundField DataField="StudentName" HeaderText="Student Name" ItemStyle-Width="150" />
    </Columns>
</asp:GridView>

Namespace

using System.Data;
using System.Configuration;
using System.Data.SqlClient;

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.PopulateStudents();
    }
}
private void PopulateStudents()
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT RollNumber, StudentName FROM Students", con))
        {
            con.Open();
            this.gvStudents.DataSource = cmd.ExecuteReader();
            this.gvStudents.DataBind();
            con.Close();
        }
    }
}
protected void DeleteStudent(object sender, EventArgs e)
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("DELETE FROM Students WHERE RollNumber = @RollNumber", con))
        {
            cmd.Parameters.AddWithValue("@RollNumber", Convert.ToInt32(this.txtRollNumber.Text.Trim()));
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }

    Response.Redirect(Request.Url.AbsoluteUri);
}

Screenshot