abstract class ListOfString {
  abstract int length();

  abstract boolean gotMilk();
  boolean notMilk() {
    return !this.gotMilk();
  }
  abstract ListOfString sour();
}
 
class EmptyListOfString extends ListOfString {
   EmptyListOfString() { }
   int length() { return 0; } 
   boolean gotMilk() { return false; }
   ListOfString sour() { return new EmptyListOfString(); }
}
 
class ConsListOfString extends ListOfString {
   String first;
   ListOfString rest;
   ConsListOfString(String first, ListOfString rest) {
     this.first = first;
     this.rest = rest;
   }
   int length() { return 1 + this.rest.length(); } 
   boolean gotMilk() { return (this.first.equals("milk") 
                               || this.rest.gotMilk());  }
   ListOfString sour() { 
    if (this.first.equals("milk"))
      return new ConsListOfString("yogurt", this.rest.sour());
    else
      return new ConsListOfString(this.first, this.rest.sour());
  }
}