SP is undefined in sharepoint hosted app client Webpart

Hello All,

For a client web part in sharepoint hosted app, I am getting the following error

"SP is undefined".

My code is bel0w

   <script
 type="text/javascript"
 src="//ajax.aspnetcdn.com/ajax/4.0/1/MicrosoftAjax.js">
</script>
    <script type="text/javascript" src="../Scripts/jquery-1.9.1.min.js"></script>
     <script  type="text/javascript" src="/_layouts/sp.core.js"></script>
    <script type="text/javascript" src="/_layouts/15/sp.runtime.js"></script>
        <script type="text/javascript" src="/_layouts/15/sp.js"></script>

    <script type="text/javascript">
        'use strict';

        $(document).ready(function () {
                    ExecuteOrDelayUntilScriptLoaded(loadConstants, "sp.js");
                });
                var  clientContext = SP.ClientContext.get_current();
               var  web = clientContext.get_web();
            });

</script>     ....

I am getting the error ExecuteOrDelayUntilScriptLoaded is undefied. This script is in client Web part

Any help is appreciated.

Vishnu