interface Surface {
   int surface_content();
}

class Square implements Surface {
   int width;
   Square(int this.width);
   int surface_content() => width*width;
}

class Rectangle extends Square {
   int _height;
   Rectangle(int this._height, int br) : super(br);
   int surface_content() => width * _height;
}

main() {
   Square q = new Square(3);
   Rectangle r = new Rectangle(2,3);
   print(q.surface_content());
   print(r.surface_content());
}