input: Write a function to calculate the square root of a given positive number using Python's math module. If the number is negative, the function should return an error message.
  
  def calculate_square_root(n):
      if n < 0:
          return "Error: Input must be a positive number."
      return math.sqrt(n)
  
  assert calculate_square_root(100) == 10.0
  assert calculate_square_root(25) == 5.0
  assert calculate_square_root(49) == 7.0
  assert calculate_square_root(-1) == "Error: Input must be a positive number."
  assert calculate_square_root(0) == 0.0
output: def calculate_square_root(n):
      if n < 0:
          return "Error: Input must be a positive number."
      return math.sqrt(n)