input: Write a function to calculate the square root of a user-defined positive number and return the result. If the number is negative, return a message indicating that the input is invalid.
  
  def calculate_square_root(n):
      if n < 0:
          return "Invalid input: Number must be positive."
      else:
          import math
          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(-16) == "Invalid input: Number must be positive."
  assert calculate_square_root(0) == 0.0
output: def calculate_square_root(n):
      if n < 0:
          return "Invalid input: Number must be positive."
      else:
          import math
          return math.sqrt(n)