def save_results_to_csv(experiment_data: list[dict], file_path: str) -> None:
    """
    Write VRPTW experimental results' data to a CSV file.

    The CSV header order is determined from the first element of
    ``experiment_data``: first all keys in ``experiment_data[0]['input']`` (in
    insertion order), then all keys in ``experiment_data[0]['output']``. Each
    subsequent row follows the same header order.

    Values are written with ``str(...)`` and are not quoted/escaped.

    Parameters
    ----------
    experiment_data : list of dict
        A list of dictionaries representing the run of a VRPTW experiment.
        Each dict has 'input' and 'output' sub-dicts.
    file_path : str
        The file path where the CSV file will be written.

    Examples
    --------
    >>> experiment_data = [
            {'input': {
                'experiment_id': 1,
                'vehicle_count': 3,
                'customer_count': 20},
             'output': {
                 'total_distance': 100,
                 'total_time': 50,
                 'service_level': 95}},
            {'input': {
                'experiment_id': 2,
                'vehicle_count': 4,
                'customer_count': 25},
             'output': {
                 'total_distance': 150,
                 'total_time': 60,
                 'service_level': 90}}]

    >>> save_results_to_csv(experiment_data, 'path/to/vrp_results.csv')

    Content of `path/to/vrp_results.csv`:

    ```csv
    experiment_id,vehicle_count,customer_count,total_distance,total_time,service_level
    1,3,20,100,50,95
    2,4,25,150,60,90
    ```
    """
    # Combine keys from input and output for headers
    raise NotImplementedError("Complete this function.")