How can I perform Mathematical calculation in RichTextEditor?

<ej:RTE ID="RTECalc" runat="server" ClientIDMode="Static">
    <Tools>
        <CustomTool>
            <ej:CustomTool Text="Math Calc" Tooltip="Click for calculation" Action="Calculate" />
        </CustomTool>
    </Tools>
    <RTEContent>
        <p><b>Description:&lt;/b&gt;&lt;/p&gt;&lt;p&gt;The Rich Text Editor (RTE) control is easy to render in client side. You can easily edit the contents and get the HTML content for the displayed content. The rich text editor control provides a toolbar that helps apply rich text formats to the text entered in the text area. &lt;/p&gt; 
    </RTEContent>
</ej:RTE>
        <%--Dialog to be displayed on CustomTool Action--%>
<ej:Dialog ID="customSourceCode" ClientIDMode="Static" Title="Math Calc" ClientSideOnClose="OnClose" ShowOnInit="false" EnableModal="true" Width="400" runat="server">
    <DialogContent>
        <table>
            <tr> <td>Data 1</td> </tr>
            <tr>
                <td colspan="2">
                    <ej:NumericTextBox runat="server" ID="num1" ClientIDMode="Static"></ej:NumericTextBox>
                </td>
            </tr>
            <tr> <td>Data 2</td> </tr>
            <tr>
                <td colspan="2">
                    <ej:NumericTextBox runat="server" ID="num2" ClientIDMode="Static"></ej:NumericTextBox>
                </td>
            </tr>
            <tr>
                <td colspan="2">
                    <div class="e-rte-button e-fieldseparate">
                        <ej:Button ID="addition" Type="Button" Text="Add" runat="server" ClientSideOnClick="OnPerformOperation"></ej:Button>
                        <ej:Button ID="multiplication" Type="Button" Text="Multiply" runat="server" ClientSideOnClick="OnPerformOperation"></ej:Button>
                        <ej:Button ID="Close" Type="Button" Text="Cancel" runat="server" ClientSideOnClick="OnPerformOperation"></ej:Button>
                    </div>
                </td>
            </tr>
        </table>
    </DialogContent>
</ej:Dialog>

<script type="text/javascript">
    function Calculate() {
        //Opens dialog.
        $("#customSourceCode").ejDialog("open");
    }
    function OnPerformOperation(args) {
        //Calculation for the corresponding math operation.
        rteObj = $("#RTECalc").data("ejRTE");
        num1 = $("#num1").data("ejNumericTextbox");
        num2 = $("#num2").data("ejNumericTextbox");
        var result;
        switch (args.model.text) {
            case "Add":
                result = "Sum of " + num1.getValue() + " and " + num2.getValue() + " is " + (num1.getValue() + num2.getValue());
                break;
            case "Multiply":
                result = "Multiplication of " + num1.getValue() + " and " + num2.getValue() + " is " + num1.getValue() * num2.getValue();
                break;
        }
       // Inserts the result to the RTE content.
        rteObj.executeCommand("inserthtml", result);
        $("#customSourceCode").ejDialog("close"); // Close the dialog
    }
</script>