An integer X (1 ≤ X ≤ 10^18), representing the lower bound.
An integer Y (X ≤ Y ≤ 10^18), representing the upper bound.

### Example Input:

```
3 20
```

### Function Signature:
Write a function f(X, Y) that takes in the input. 
def f(X: int, Y: int): 
    ''' 
    X: an integer
    Y: an integer
    '''