def get_key_w_nested_folder(keys):
    """
    Given a list of S3 keys, returns the first full key that contains
    a nested folder.

    Args:
        keys (list<str>): A list of candidate S3 keys
    Returns:
        str: The prefix of the first key found to contain nested folders
    """
    for key in keys:
        if key.count('/') > 1:
            return key