input: Write a function to calculate the square root of a given positive number. If the input is not a positive number, return None.
  assert calculate_square_root(100)==10
  assert calculate_square_root(4)==2
  assert calculate_square_root(0)==None
  assert calculate_square_root(-9)==None
  assert calculate_square_root(25)==5
output: def calculate_square_root(n):
      if n < 0:
          return None
      return n ** 0.5