input: Write a python function to calculate the amount of loss when the given value shows 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(selling_price, buying_price):
      loss = buying_price - selling_price
      if loss > 0:
          return loss
      return None