问题描述:
Write a function that gives loss amount if the given amount has loss else return none.

测试用例:
['assert loss_amount(1500,1200)==None', 'assert loss_amount(100,200)==100', 'assert loss_amount(2000,5000)==3000']

生成的代码:
def loss_amount(initial_amount, current_amount):
    if current_amount < initial_amount:
        return initial_amount - current_amount
    else:
        return None