input: Write a python function that returns the loss if the provided amount incurs a loss, otherwise return None.
  assert calculate_loss(1500, 1200) == None
  assert calculate_loss(100, 200) == 100
  assert calculate_loss(2000, 5000) == 3000
output: def calculate_loss(amount, cost):
      loss = cost - amount
      if loss > 0:
          return loss
      return None