input: Write a python function to generate a random number between a given range.
  assert 0 <= random_num(0, 10) <= 10
  assert 50 <= random_num(50, 100) <= 100
  assert -10 <= random_num(-10, 10) <= 10
output: import random
  
  def random_num(start, end):
      return random.randint(start, end)