function isNumberKey(evt)
      {
	 var charCode = (evt.which) ? evt.which : event.keyCode
	 if (charCode !=8 && (charCode < 45 || charCode > 57))
            return false;

         return true;
	}
	
function mmg()
{
var a = document.form1.a.value;
var b = document.form1.b.value;
if((a=="" && b=="")||(a=="" || b==""))
{
alert("please enter values for all fields");
}
else
{
var c=(1/a);
var d=(1/b);
var e=(eval(c)+eval(d));
document.form1.e.value=e;
}
}