import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

class lista{
    String val;
    lista st,dr;
    public lista(String val1) {
        val=val1;

    }
    public lista() {}
}

public class p1 {

    static void citire(lista x) throws IOException{

    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    String text=br.readLine();
    x.val=text;
    if(text.compareTo("f") !=0) {
        x.st=new lista();
        x.dr=new lista();
        citire(x.st);
        citire(x.dr);
    }

    }

    static void afisare_preorder(lista x){
        System.out.println(x.val);
        if(x.st!=null) afisare_preorder(x.st);
        else {
            System.out.println(x.val);
            x=x.dr;
        }
    }


    public static void main(String[] args) throws IOException {
        lista nod=new lista("abc");
        citire(nod);
        afisare_preorder(nod);


    }

}