input: Write a python function to prompt the user to enter a valid number and return it as an integer.
  assert type(get_valid_number())==int
  assert get_valid_number() > 0
output: def get_valid_number():
      while True:
          try:
              number = int(input("Enter a valid number: "))
              if number > 0:
                  return number
              else:
                  print("Number must be greater than 0.")
          except ValueError:
              print("Invalid input. Please enter a valid integer.")