def build_fgts_cash_flow(fgts_amount, fgts_frequency, n_months):
    """
    Build a dict representing times and amounts ({month: amount}) where fgts will be used.
    """
    fgts_payments = {}

    for i in range(1, n_months + 1):

        if i % (fgts_frequency * 12) == 0:
            fgts_payments[i] = fgts_amount
        else:
            fgts_payments[i] = 0

    return fgts_payments