Error: Specified argument was out of the range of valid values in C#

Hi everyone. I have problem. I want price in gridview rows calculate. But only one column not calculate.

 Html Code

<asp:GridView ID="example" CssClass="table table-striped table-bordered table-hover"
    AutoGenerateColumns="false" DataKeyNames="ID" runat="server" ClientIDMode="Static">
    <Columns>
        <asp:TemplateField HeaderText="Ürün Ad" ItemStyle-Width="100">
            <ItemTemplate>
                <asp:Label ID="lblUrun" runat="server" Text='<%# Eval("UrunAd") %>'></asp:Label>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="txturunad" runat="server" Text='<%# Eval("UrunAd") %>'></asp:TextBox>
            </EditItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Stok" ItemStyle-Width="50">
            <ItemTemplate>
                <asp:Label ID="lblStok" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("Stok") %>'></asp:Label>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="txtstok" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("Stok") %>'></asp:TextBox>
            </EditItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Fiyat" ItemStyle-Width="50">
            <ItemTemplate>
                <asp:TextBox ID="txtfiyat" Style="padding: 3px 5px; width: 100%" CssClass="form-control"
                    runat="server"></asp:TextBox>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="txtfiyat" Style="padding: 3px 5px; width: 100%" runat="server" CssClass="form-control"></asp:TextBox>
            </EditItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Tedarikçi" ItemStyle-Width="50">
            <ItemTemplate>
                <asp:Label ID="lblTedarikci" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("TedarikciAd") %>'></asp:Label>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="txtTedarikci" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("TedarikciAd") %>'></asp:TextBox>
            </EditItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Proje" ItemStyle-Width="50">
            <ItemTemplate>
                <asp:Label ID="lblProje" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("ProjeAd") %>'></asp:Label>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="txtProje" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("ProjeAd") %>'></asp:TextBox>
            </EditItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Eleman Fiyat" ItemStyle-Width="50">
            <ItemTemplate>
                <asp:TextBox ID="txtElemanFiyat" Style="padding: 3px 5px; width: 100%" CssClass="form-control"
                    runat="server"></asp:TextBox>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="txtEeefiyat" runat="server" Style="padding: 3px 5px; width: 100%"
                    CssClass="form-control"></asp:TextBox>
            </EditItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>

Code Behind

foreach (GridViewRow rw in example.Rows)
{
    Label lb = (Label)rw.Cells[1].FindControl("lblUrun");

    Label lb1 = (Label)rw.Cells[2].FindControl("lblStok");

    int stok = Convert.ToInt32(lb1.Text);

    TextBox txtfiyat = (TextBox)rw.Cells[3].FindControl("txtfiyat");

    int adet = Convert.ToInt32(txtfiyat.Text);

    //DropDownList epoz = (DropDownList)rw.Cells[4].FindControl("drpEleman");

    Label eadet = (Label)rw.Cells[4].FindControl("lblTedarikci");

    Label proje = (Label)rw.Cells[5].FindControl("lblProje");

    TextBox efiyat = (TextBox)rw.Cells[6].FindControl("txtElemanFiyat");

    int eadetz = Convert.ToInt32(efiyat.Text);

    if (txtfiyat.Text == "")
    {
        Response.Write("<script LANGUAGE='JavaScript' >alert('Fiyat Boş bırakmayınız...')</script>");
        break;
    }


    int sayi1, sayi2, sonuc;

    int sayi3, sayi4, sonuc2;

    sayi1 = adet;
    sayi2 = Convert.ToInt32(txtToplamfiyat.Text);

    sonuc = sayi1 + sayi2;

    sayi3 = eadetz;
    sayi4 = Convert.ToInt32(txtElemanToplamFiyat.Text);

    sonuc2 = sayi3 + sayi4;

    txtToplamfiyat.Text = sonuc.ToString();

    txtElemanToplamFiyat.Text = sonuc2.ToString();
}