input: Write a python function to calculate BMI given height and weight.
  assert calculate_bmi(1.75, 70) == 22.857142857142858
  assert calculate_bmi(1.80, 80) == 24.691358024691358
  assert calculate_bmi(1.60, 50) == 19.53125
output: def calculate_bmi(height, weight):
      return weight / (height ** 2)