An integer N (1 ≤ N ≤ 1000), representing some quantity or size.
An integer M (1 ≤ M ≤ N), representing some quantity or size.
A list of integers A of size M, where each integer is between 1 and N and A is in strictly increasing order.

### Example Input:

```
6 4
1 4 5 6
```

### Function Signature:
Write a function f(N, M, A) that takes in the input.
def f(N: int, M: int, A: List[int]):
    '''
    N: an integer
    M: an integer
    A: a list of integers
    '''