input: Write a python function to generate a random number within a given range.
  assert random_num(1, 10) in range(1, 11)
  assert random_num(50, 100) in range(50, 101)
  assert random_num(-10, 10) in range(-10, 11)
output: import random
  
  def random_num(start, end):
      return random.randint(start, end)