Hi, here's your problem today. This problem was recently asked by Microsoft:

Given two strings which represent non-negative integers, multiply the two numbers and return the product as a string as well. You should assume that the numbers may be sufficiently large such that the built-in integer type will not be able to store the input (Python does have BigNum, but assume it does not exist).

Here's an example and some starter code.

```
def multiply(str1, str2):
# Fill this in.
print(multiply("11", "13"))
# 143
```