An integer N (2 ≤ N ≤ 2 × 10^5), representing the length of a string.
A string S of length N, consisting of characters {'a', 'b', 'c'}.

### Example Input:

```
5
abbac
```

### Function Signature:
Write a function f(N, S) that takes in the input.
```python
def f(N: int, S: str):
    ''' 
    N: an integer 
    S: a string
    '''
```