input: Write a python function to calculate the area of a circle given its radius.
  assert get_area(1) == math.pi
  assert get_area(2) == 4 * math.pi
  assert get_area(0) == 0
  assert get_area(3.5) == math.pi * 3.5**2
output: import math
  
  def get_area(radius):
      return math.pi * radius ** 2