class SyntaxAnalyse {
    public static final boolean DEBUG = true;
	
	private String  expressionS = null; // enthaelt die Expression
	public  int     pos  = 0;           // aktuelle Position in expressionS
	public  char    next = ' ';         // naechstes Zeichen
	private boolean eof  = false;       // signalisiert Lese-Ende
	
	/**
	 *  Konstruktor: uebergeben wird der String, der analysiert werden soll
	 */
	public SyntaxAnalyse(String s) {
		expressionS = s;
		scan();
	}
	
	/**
	 *  zeigt an, ob das Ende des Ausdrucks erreicht wurde
	 */
	public boolean eof() {
		return eof;
	}
	
	/**
	 *  kopiert das naechste Zeichen nach next
	 */
	public void scan() {
		while(!eof()) {
			if ( pos < expressionS.length() ) {
				next = expressionS.charAt(pos); 
				pos++;
			} else {
				eof = true;
				next = ' ';
			}
			if (next == ' ') continue; // Leerzeichen ueberlesen
			else             break;
		}
	}
	
	/**
	 *  prueft auf Atom (Folge von Ziffern)
	 */
	public boolean isAtom() {
		String atom = "";
		while(!eof() && next >= '0' && next <= '9') {
			atom += next;
			scan();
		}
		if (atom.length() == 0)
			return false;
		else {
			if(DEBUG)System.out.println(" Atom '"+atom+"' recognized!");
			return true;
		}		
	}
	
	/**
	 *  prueft auf Term
	 */
	public boolean isTerm() {
		if ( next=='(' ) {
			scan();
			if ( isExpression() ) {
				if ( next==')' ) {
					scan();
					if(DEBUG)System.out.println(" '(' Expression ')' recognized!");
					return true;
				} else {
					System.out.println(" Syntax Error: Missing ')' in Term!");
					return false;
				}
			} else {
				System.out.println(" Syntax Error: Missing Expression"+
								   " followed by '('!"+next);
				return false;
			}
		} else {
			return isAtom();
		}
	}
								   
	/**
	 *  prueft auf Expression
	 */
	public boolean isExpression() {
		if (isTerm()) {
			switch (next) {
				case '+':
				case '-':
					if(DEBUG)System.out.println(" Operator "+next+
												" in Expression recognized!");
					scan();
					return isExpression();
			}
			return true; // default
		} else {
			return false;
		}
	}
	
	/**
	 *  starte Syntaxanalyse
	 */
	public boolean solve() {
		return isExpression() && eof();		
	}

	public static void main(String[] argv) {
		String inputString = "help";	
		java.io.BufferedReader keyboard =
			new java.io.BufferedReader(new java.io.InputStreamReader(System.in));
		while (!inputString.equals("exit")) {
			if (inputString.equals("help")) {
				System.out.println();			
				System.out.println(" Expression ::= Term                |");
				System.out.println("                Term '+' Expression |");
				System.out.println("                Term '-' Expression ");
				System.out.println("       Term ::= Atom                |");
				System.out.println("                '(' Expression ')'");
				System.out.println("       Atom ::= <a_number>");
			}
			System.out.print("\n\nType in your Expression, 'help' or 'exit': ");
			// read line from keyboard
			try {
				inputString = keyboard.readLine();
			}
			catch (java.io.IOException ioe) {
				System.out.println(" Error: "+ioe);				
			}
			if (inputString.equals("exit")) break;
			if (inputString.equals("help")) continue;
			if (inputString.length() == 0) continue;
			// syntax analyse 
			SyntaxAnalyse sa = new SyntaxAnalyse(inputString);
			if (sa.solve())
				System.out.println("\n\n RESULT: This is a VALID expression!");
			else
				System.out.println("\n\n RESULT: SYNTAX ERROR!");
		}
		System.out.println("Bye!");
	}

}