While loop inside a function : The problem

Write a function add_up that adds integers provided by the user, stopping when the user writes "Stop".

Conditions:
- it has to be user input, so no parameters
- nothing is told about what to do with other strings that cannot be converted to integers

My answer:

def add_up():
    `string = 0`
     total = 0
    `while string is not "Stop":`
    `string = int(input())`    
    `total += string`    
    `print(total)
add_up()