def get_solution_file_name(file_name):
    """
    Return the name of the file in which save the solution

    :param file_name: the name of the file.
    :return: A .sol filename
    :rtype: str
    """
    if file_name is None:
        return None

    if file_name.endswith('.sol'):
        return file_name
    else:
        return f"{file_name}.sol"