function calculatePrice(numAttendees, priceFirst, priceAdditional) {
  var price = 0;
  if (numAttendees != 0){
    if (numAttendees == 1) {
      price = priceFirst;
    }
    if (numAttendees == 2) {
      price = priceFirst;
    }
    if (numAttendees == 3) {
      price = priceFirst;
    }
    if (numAttendees > 3) {
      price = priceFirst + (priceAdditional * (numAttendees - 3));
    }
  }
  return price;
}
function updatePrice(form){
  var priceFirst = 1350;
  var priceAdditional = 300;
  var price = calculatePrice(form.numAttendees.value,priceFirst,priceAdditional);
  var show_price = price + ".00";
  $('paypal').amount.value = show_price;
  $('seminar_price').innerHTML = "Seminar Price ... " + "$" + show_price;
}
  
window.addEvent ('domready', function () {
  updatePrice($('numAttendeesForm'));
    var check = new FormCheck('numAttendeesForm', {trimValue: true});
  $('numAttendeesForm').addEvent('submit', function(event) {
                                   if ($('numAttendeesForm').isValid){
                                     event.stop();
                                     updatePrice($('numAttendeesForm'));
                                   }
                                 });
});