import java.util.*;
import java.io.*;

class UnusedContentException extends Exception {
    public UnusedContentException(String message){
        super(message);
    }
}

class Aligner {
    private int depth;
    private boolean isBlock;
    private boolean callAlign;

    public Aligner(){
        this(0, false, false);
    }
    public Aligner(int depth, boolean isBlock){
        this(depth, isBlock, true);
    }
    public Aligner(int depth, boolean isBlock, boolean callAlign){
        this.depth = depth;
        this.isBlock = isBlock;
        this.callAlign = callAlign;
    }

    public int getDepth(){
        return depth;
    }
    public boolean getIsBlock(){
        return isBlock;
    }
    public boolean shouldAlign(){
        return callAlign;
    }

    private String getPrefix(){
        String prefix = "";
        for(int j = depth; j > 0; --j){
            prefix += "    ";
        }
        return prefix;
    }


    public String alignTag(String mid){
        return alignTag(mid, false);
    }
    public String alignTag(String mid, boolean isBlock){
        String str = "\n";
        String prefix = getPrefix();
        str += prefix;
        int index = mid.lastIndexOf("</");
        if ((index > -1) && (isBlock))
            str += mid.substring(0, index) + prefix + mid.substring(index);
        else
            str += mid;
        str += "\n";
        return str.replace("\n\n", "\n");
    }
}

class Tag {

    private String name;
    private String content;
    private ArrayList<Tag> elements = new ArrayList<>();

    public Tag(String name){
        this(name, "");
    }
    public Tag(String name, String content){
        this.name = name;
        this.content = content;
    }
    public Tag(Tag other){
        this(other.name, other.content);
        this.elements = new ArrayList<>(other.elements);
    }

    public String getName(){
        return name;
    }
    public String getContent(){
        return content;
    }

    public Tag getElement(int i){
        return elements.get(i);
    }
    public ArrayList<Tag> getElements(){
        return elements;
    }

    public Tag add(String name){
        Tag temp = new Tag(name);
        elements.add(temp);
        return temp;
    }
    public Tag add(Tag inner){
        Tag temp = new Tag(inner);
        elements.add(temp);
        return temp;
    }



    private String prepareTag(String mid){
        String str = "";
        str += ((name.length() > 0) ? ("<" + name + ">") : "");
        str += mid;
        str += ((name.length() > 0) ? ("</" + name + ">") : "");
        return str;
    }

    public String export() throws UnusedContentException {
        return export(new Aligner());
    }
    public String export(int depth) throws UnusedContentException {
        return export(new Aligner(depth, true));
    }
    private String export(Aligner a) throws UnusedContentException {
        if (elements.size() == 0){
            String strToAlign = (content != "") ? prepareTag(content) : ("<" + name + " />");
            return a.shouldAlign() ? a.alignTag(strToAlign) : strToAlign;
        } else if (content == ""){
            String mid = "";
            for(int i = 0; i < elements.size(); ++i){
                Aligner aa = new Aligner(a.getDepth()+1, a.getIsBlock(), a.shouldAlign());
                mid += elements.get(i).export(aa);

            }
            String strToAlign = prepareTag(mid);
            return a.shouldAlign() ? a.alignTag(strToAlign, a.getIsBlock()) : strToAlign;
        } else {
            throw new UnusedContentException("Unused content!");
        }
    }

}

public class Sandbox {
    public static void main(String args[]) {
        try {

            Tag myPage = new Tag("html");
            Tag myHead = myPage.add("head");
            myHead.add(new Tag("title", "Hello world!"));

            Tag myBody = myPage.add("body");
            myBody.add(new Tag("h1", "My heading"));
            myBody.add(new Tag("p", "Lorem ipsum"));

            Tag myPar = myBody.add(new Tag("p", ""));
            myPar.add(new Tag("", "My paragraph"));

            Tag myLink = new Tag("a");
            Tag myImg = myLink.add(new Tag("img", ""));
            Tag myTxt = myLink.add(new Tag("", "My link"));

            Tag myDiv1 = new Tag("div");
            Tag myDiv2 = myDiv1.add(new Tag("div"));
            Tag myDiv3 = myDiv2.add(new Tag("div"));
            Tag myDiv4 = myDiv3.add(new Tag("div", "Tekstas tekstas " + myLink.export() + " tekstas tekstas"));
            myBody.add(myDiv1);

            System.out.println(myPage.export(0));

            /*
            FileWriter fw = new FileWriter("index.htm");
            fw.write(myPage.export());
            fw.close();
            */

        } catch(Exception e){
            e.printStackTrace();
            System.out.println("Unexpected error, sorry!");
        }
    }
}