一、规则3：最好的提示编程能力和解决问题能力的方式就是练习。（Rule 3: The best way to improve your programming and problem skills is to practice!.）

# Code Listing 1.1
# Calculate the area and circumference of a circle from its radius.
# Step 1: Prompt for a radius.
# Step 2: Apply the area formula.
# Step 3: Print out the results.
import math
radius_str = input("Enter the radius of your circle: ")
radius_int = int(radius_str)
circumference = 2 * math.pi * radius_int
area = math.pi * (radius_int ** 2)
print ("The cirumference is:",circumference, "and the area is:",area)