input: Write a function to calculate the square root of a user-defined positive number using the math module. The function should return the square root of the given number. If the number is negative, return an error message.
  assert square_root(100) == 10.0
  assert square_root(25) == 5.0
  assert square_root(-1) == "Error: Number must be positive."
  assert square_root(0) == 0.0
output: import math
  
  def square_root(num):
      if num < 0:
          return "Error: Number must be positive."
      return math.sqrt(num)