function getQuerystring(key, default_)
{
  if (default_==null) default_="";
  key = key.replace(/[\[]/,"\\\[").replace(/[\]]/,"\\\]");
  var regex = new RegExp("[\\?&]"+key+"=([^&#]*)");
  var qs = regex.exec(window.location.href);
  if(qs == null)
    return default_;
  else
    return qs[1];
}

function setAmount(){
  var amount = getQuerystring("a");

  if(amount != null && amount != ""){
    document.frmMain.quantity.value = amount;
  }
}

window.onload = setAmount;