input: Write a function to calculate the square root of a given positive number. If the input number is less than 0, return -1 to indicate invalid input.
  
  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(0)==0.0
  assert calculate_square_root(-1)==-1
output: def calculate_square_root(num):
      if num < 0:
          return -1
      return num ** 0.5