Input string was not in a correct format.

Coding of First Page is:

    protected void Button1_Click(object sender, EventArgs e)
    {
        Response.Redirect("../AddCart.aspx?rate=" + this.Label1.Text + "&qty=" + this.DropDownList1.SelectedValue);
    }
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Coding of Second Page is: (Error here in this page)

protected void Page_Load(object sender, EventArgs e)
    {

        this.Label1.Text = Request.QueryString["rate"];
        this.Label2.Text = Request.QueryString["qty"];

        int rat = Convert.ToInt32(Label1.Text);
        int qt = Convert.ToInt32(Label2.Text);
     ...