function check_email(email) {
   invalid = " /:,;";
   if(email == "") { return false }
   for(i=0; i < invalid.length; i++) {
      bad = invalid.charAt(i);
      if (email.indexOf (bad, 0) != -1) { return false }
   }
   at = email.indexOf("@", 0);
   if( (at == -1) || (at == 0) ) { return false }
   if(email.indexOf("@", at+1) != -1) { return false }
   period = email.indexOf(".", at);
   if( (period == -1) || (period == at+1) ) { return false }
   if(period+3 > email.length == -1) { return false }
   return true;
}
function sprawdz(f) {
   if (!(check_email(f))) {
      return false;
   }
   else {
      return true;
   }
}