How to Disable Paste option for alphabets and Special Characters but Enabled for Numbers

I Used this code for restricting to type alphabets and characters.

  <script type="text/javascript">
    function isNumber(evt) {
        evt = (evt) ? evt : window.event;
        var charCode = (evt.which) ? evt.which : evt.keyCode;
        if (charCode > 31 && (charCode < 48 || charCode > 57)) {
            return false;
        }
        return true;
    }

</script>
<asp:TextBox ID="txtCode" runat="server" Width="15%"   onkeypress="return isNumber(event)"/>

But I am able to Paste the alphabets and characters in it(Which I don't want).

When I used this code, The Paste Option is disabled but,I cant Paste even Numbers

   <asp:TextBox ID="txtCode" runat="server" Width="15%" ondrop="return false;"
     onpaste="return false;" onkeypress="return isNumber(event)" />

But I want to paste the Numbers :(,How can I do that ?.