input: Write a python function to calculate the area of a circle.
  assert area_of_circle(1) == 3.141592653589793
  assert area_of_circle(0) == 0.0
  assert area_of_circle(2.5) == 19.634954084936208
output: def area_of_circle(radius):
      import math
      return math.pi * radius ** 2