input: Write a python function to prompt the user to enter a number and return the number as an integer.
  assert get_user_input() == 5  # if user inputs '5'
  assert get_user_input() == 10  # if user inputs '10'
  assert get_user_input() == -3  # if user inputs '-3'
output: def get_user_input():
      return int(input("Enter a number: "))