def forgetting_to_bwt(f):
    """
    Convert forgetting to backward transfer.
    BWT = -1 * forgetting
    """
    if f is None:
        return f
    if isinstance(f, dict):
        bwt = {k: -1 * v for k, v in f.items()}
    elif isinstance(f, float):
        bwt = -1 * f
    else:
        raise ValueError("Forgetting data type not recognized when converting"
                         "to backward transfer.")
    return bwt