ASP.Net Error: Non-invocable member System.Configuration.ConfigurationManager.ConnectionStrings cannot be used like a method

hi, i am trying to build a connection string in web config but still connection is not build.

Error

Non-invocable member 'System.Configuration.ConfigurationManager.ConnectionStrings' cannot be used like a method

<connectionStrings>
    <add name="hotelManagement" connectionString="server = LENOVO-PC\himani;database=hotel_management;Integrated Security=True;User Instance=True"></add>
</connectionStrings>

public partial class _Default : System.Web.UI.Page 
{
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        string username = txtUserName.Text;
        string password = txtPassword.Text;
        //string cs = "data source = ,;database=hotel_management;";
        string cmg = ConfigurationManager.ConnectionStrings("hotelManagement").ConnectionString;
        SqlConnection cnn = new SqlConnection(cmg);
        cnn.Open();
        string query = "sp_insertdata";
        SqlCommand cmd = new SqlCommand(query,cnn);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("username",username);
        cmd.Parameters.AddWithValue("password",password);
        cmd.ExecuteNonQuery();
        Response.Write("account created successfully");
        cnn.Close();

    }
}

can you tell me what's wrong with my code.