Canonicalized Input Description: 

A string s consisting of lowercase English letters with length between 1 and 500,000.
A string t consisting of lowercase English letters with length between 1 and 500,000.

### Example Input:

```
abcabab
ab
```

### Function Signature:
Write a function f(s, t) that takes in the input.
```python
def f(s: str, t: str):
    '''
    s: a string
    t: a string
    '''
```