input: Write a function that calculates the loss amount if the provided amount has decreased, otherwise return None.
  assert loss_amount(1500, 1200) == None
  assert loss_amount(100, 200) == 100
  assert loss_amount(2000, 5000) == 3000
output: def loss_amount(current_amount, amount_after):
      if amount_after < current_amount:
          return current_amount - amount_after
      return None