Can't figure out the javascript

Hello i am at the moment making a javascript temperature converter where the user keys in a number and can convert it to either Celsius or Fahrenheit by clicking on either one of the radio buttons. I'm not sure how to code it in so it displays this for the user.

Here is my code so far,


<!-- Temperature Converter -->

<html>

 <head>

  <title>Temperature Converter</title>

  <style type="text/css">

   body {

          background-color: #000000;

          color: #FFFFFF;

          text-align: center;

          font-size: 25;

        }

  </style>


  <script type="text/javascript">

    

    function convert()

  {


  if (document.frm.txtTemp.value == "")

         {

            alert("Please enter a temperature");

            document.frm.txtTemp.focus();

         }

         else if (document.frm.txtTemp.value != parseFloat(document.frm.txtTemp.value))

         {

            alert("Your temperature must be a number");

            document.frm.txtTemp.focus();

         }

         else if (document.frm.Cels.Faren.value == "")

         {

            alert("Please select one of the radio buttons");

            document.frm.Cels.Faren.focus();

         }


  }


  function clean()

       {

         //alert("in clean");

         document.frm.txtTemp.value = "";

         document.frm.txtTemp.value = "";

         document.frm.readonly.value = "";

         document.frm.txtTemp.focus();

         document.frm.txtTemp.focus();

       }


   </script>


 </head>

 <body onload="window.document.frm.txtTemp.focus()">


  <h1>Temperature Converter</h1>

  <form name="frm"> 

    <tr><td>Enter a temperature:  </td>

    <td><input type=text name="txtTemp" size=12"></td>

    <br /><br />

    <td>

    <input type="radio" name="Cels" value="Celsius" class="radio" /> Celsius<br />

    <input type="radio" name="Faren" value="Fahrenheit" class="radio" /> Fahrenheit

    </td>


    <br /><br />

    <input type="button" name="cmdConvert" value="Convert" onclick="convert()">

    	 

    <input type="button" name="cmdClear" value="Reset" onClick="clean()">

    <br /><br /><br />

    <input type=text name="readonly" size="12" disabled="READONLY" />

   </form>


 </body>

</html>

Many thanks to whoever can help me.

else if (!(document.frm.Cels.checked || document.frm.Faren.checked)) { alert("Please select one of the radio buttons"); document.frm.Cels.Faren.focus(); } else { if(document.frm.Cels.checked) { var fahrenheit = parseFloat(document.frm.txtTemp.value); document.frm.readonly.value = (fahrenheit - 32 ) * (5/9); } if(document.frm.Faren.checked) { var celcius = parseFloat(document.frm.txtTemp.value); document.frm.readonly.value = (celcius * 9/5 ) +32; } }