An integer h (2 ≤ h ≤ 30), an integer w (2 ≤ w ≤ 30), and an integer n (2 ≤ n ≤ 30).
A list of h strings, each containing exactly w characters, where each character is either an uppercase letter ('A'-'Z') or a period ('.').

### Example Input:

```
4 6 3
......
...Y..
...Y..
RRYRY.
```

### Function Signature:
Write a function f(h, w, n, field) that takes in the input.
def f(h: int, w: int, n: int, field: List[str]):
    '''
    h: an integer
    w: an integer
    n: an integer
    field: a list of strings
    '''