Find whether TextBox value is valid and positive integer in C# .Net

This Way:

<form id="form1" runat="server">
    <div>
        <asp:TextBox ID="txtNumber" runat="server" />
        <asp:Button ID="btnSubmitNumber" runat="server" OnClick="SubmitNumber" Text="Submit Number" />
    </div>
</form>

C#:

protected void SubmitNumber(object sender, EventArgs e)
    {
        if (string.IsNullOrEmpty(this.txtNumber.Text.Trim()) || Convert.ToInt32(this.txtNumber.Text.Trim()) < 0)
        {
            ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", "alert('Number is not correct')", true);
        }        
    }

Thank You.