The below code use to add twoo number by the using of JavaScript only.

<script language="javascript" type="text/javascript">

function Navigate() {

var x = document.getElementById("txt1").value;

var y = document.getElementById("txt2").value;

var z =parseInt( x )+parseInt( y);

document.getElementById("txt3").value = z;

}

</script>

<body>

<form id="form1" runat="server">

<div>

<%-- <asp button type="button" onclick="alert('Welcome!')">Click Me!</button>--%>

<asp:Button ID="Button1" UseSubmitBehavior="true" Text="Open Web site" OnClientClick="Navigate()"

runat="server" />

<asp:TextBox ID="txt1" runat="server"></asp:TextBox>

<asp:TextBox ID="txt2" runat="server"></asp:TextBox>

<asp:TextBox ID="txt3" runat="server"></asp:TextBox>

</div>

</form>