### Input Description:

- An integer \( n \) (1 ≤ \( n \) ≤ 10^8), representing some length or size.
- An integer \( k \) (1 ≤ \( k \) ≤ \( n \)), representing an index position.
- An integer \( x0 \) (0 ≤ \( x0 \) ≤ 9), representing an initial value.

### Example Input:
```
20 10 1
```

### Function Signature:
```python
def f(n: int, k: int, x0: int):
    '''
    n: an integer
    k: an integer
    x0: an integer
    '''
```