input: Write a function to generate a random number between the given start and stop input numbers. If the input values are invalid, return -1.
  assert random_num(0, 10) >= 0 and random_num(0, 10) <= 10
  assert random_num('a', 10) == -1
  assert random_num(10, 0) == -1
  assert random_num(-5, 5) >= -5 and random_num(-5, 5) <= 5
output: def random_num(start, stop):
      if not isinstance(start, int) or not isinstance(stop, int) or start > stop:
          return -1
      import random
      return random.randint(start, stop)