import java.util.ArrayList;
import java.util.List;

public class Main2 {
	
	public static void main(String[] args) {
		Square s1 = new Square(4);
		Square s2 = new Square(9);
		Rectangle r1 = new Rectangle(3, 4);
		Rectangle r2 = new Rectangle(5, 6);
		
		List<Shape> shapes = new ArrayList<Shape>();
		shapes.add(s1);
		shapes.add(s2);
		shapes.add(r1);
		shapes.add(r2);
		
		List<Square> squareShapes = new ArrayList<Square>();
		squareShapes.add(s1);
		squareShapes.add(s2);
		
		List<Rectangle> rectangleShapes = new ArrayList<Rectangle>();
		rectangleShapes.add(r1);
		rectangleShapes.add(r2);
		
		Main2 m = new Main2();
		
		double avgArea = m.averageArea(shapes);
		System.out.println(avgArea);
		
		double avgSquareArea = m.averageArea(squareShapes);
		System.out.println(avgSquareArea);
		
		double avgRectangleArea = m.averageArea(rectangleShapes);
		System.out.println(avgRectangleArea);
	}
	
	private double averageArea(List<? extends Shape> shapes) {
		double sum = 0;
		for (Shape s : shapes) {
			sum += s.area();
		}
		return sum / shapes.size();
	}
}

abstract class Shape {
	
	public abstract double area();
}

class Square extends Shape {
	
	public double sideLength;
	
	public Square(double sideLength) {
		this.sideLength = sideLength;
	}
	
	public double area() {
		return sideLength * sideLength;
	}
}

class Rectangle extends Shape {
	
	public double xLength, yLength;
	
	public Rectangle(double xLength, double yLength) {
		this.xLength = xLength;
		this.yLength = yLength;
	}
	
	public double area() {
		return xLength * yLength;
	}
}