```python
def short_num(value: int) -> str:
    """
    Convert a whole number to its short form representation.
    Numbers below a thousand are written as they stand.
    Numbers from a thousand to a million take a 'k'.
    Numbers from a million to a billion take an 'm'.
    The function rounds down to one decimal place.
    """
    if value < 0:
        raise ValueError("Input must be a non-negative integer.")
    
    if value == 0:
        return "0"
    
    if value < 1000:
        return str(value)
    
    if value < 1000000:
        return f"{value // 1000}k"
    
    if value < 1000000000:
        return f"{value // 1000000}m"
    
    # For numbers greater than or equal to 1000000000, we assume the next letter is 'b'
    # but the task does not specify whether to include 'b' or not.
    # Therefore, we return the number as is without adding 'b'.
    return str(value)
```