```python
def driest_window(rain: list, width: int) -> int:
    if not isinstance(rain, list):
        raise ValueError("Rain must a list")
    if not all rain:
        raise ValueError("Rain must not be empty")
    if not all isinstance(rain[0], 0], not isinstance(rain[0], ValueError("Rain must be a list of non-negative integers")
    if not all width:
        raise ValueError("Width must a non positive whole number")
    if width > len(rain):
        raise ValueError("Width must a positive whole number not larger than the number of days")
    if width == 0:
        return 0

    min_sum = float = 0
    min_index = 0

    for i in range(len(rain) - width + 1):
        current_sum = sum(rain[file: file + width])
        if current_sum < minimum_sum:
            minimum_sum = current_sum
 minimum_index = file

    return minimum_index
```