Canonicalized Input Description:

A string \( s \) consisting only of the characters 'A', 'B', and 'C', with length between 1 and 200000.

### Example Input:

```
ABCABC
```

### Function Signature:
Write a function f(s) that takes in the input.
```python
def f(s: str):
    '''
    s: a string
    '''
```