def natural_sort_key(key):
    """ Return the given key as an (int, str) tuple with any leading integer digits as the first element, and the trailing part of the
        string as the second. A key that does not being with a digit will be returned with -1 as the first element.
    """
    int_key = ''
    for ch in key:
        if ch.isdigit():
            int_key += ch
        else:
            break
    str_key = key[len(int_key):]
    return -1 if len(int_key) == 0 else int(int_key), str_key
