interface IExpressable {
    public int interpret(final HashMap<String, Integer> variables);
}

class Plus implements IExpressable {
  private IExpressable leftOperand = null;
  private IExpressable rightOperand = null;

  public Plus(final IExpressable left, final IExpressable right) {
    leftOperand  = left;
    rightOperand = right;
  }

  @Override
  public int interpret(final HashMap<String, Integer> variables) {
    return leftOperand.interpret(variables)
            + rightOperand.interpret(variables);
  }

  public String toString() {
    return leftOperand.toString() + " + " + rightOperand.toString();
  }
}

class Parser {
  static public IExpressable parseExpression(final String expression) {
    IExpressable syntaxTree = null;
    Pattern numberPattern = Pattern.compile("[+-]?\\d+");

    Stack<IExpressable> expressionStack = new Stack<IExpressable>();
    for(String token : expression.split(" ")) {
      if(token.equals("+")) {
        IExpressable subExpression = new Plus(expressionStack.pop(), expressionStack.pop());
        expressionStack.push(subExpression);
      } else {
        expressionStack.push(new Variable(token));
      }
    }
    syntaxTree = expressionStack.pop();
    return syntaxTree;
  }
}