function check() {
    var finish="true";
    var checkdate="true";
    if(document.search_form.dest.value=="") {
     if(document.search_form.date1.value=="") {
      if(document.search_form.date2.value=="") {
       if(document.search_form.ship.value=="") {
        if(document.search_form.port.value=="") {
         if(document.search_form.duration.value=="") {
          if(document.search_form.special_search.checked == false) {
            var finish = "false";
          }
         }
        }
       }
      }
     }
    }
    if(document.search_form.date1.value!="") {
      if(document.search_form.date1.value>document.search_form.date2.value) {
        checkdate = "false";
      }
    }
    if (checkdate=="false") {
      alert ("Por favor verifique la fecha.");
    } else {
      if(finish=="false") {
        alert ("Especifique las condiciones de la búsqueda.")
      } else {
        document.search_form.submit();
      }
    }
  }