public int getPrice(long duration){
        if (this.tariff != null){
            if (tariff.getFreeMinutes() > TimeUnit.MILLISECONDS.toMinutes(duration)) return 0;
            else if (tariff.getPrepaidMinutes() + tariff.getFreeMinutes() > TimeUnit.MILLISECONDS.toMinutes(duration)) return tariff.getPrepaymentAmount();
            return (int) ((TimeUnit.MILLISECONDS.toMinutes(duration) - (tariff.getPrepaidMinutes() + tariff.getFreeMinutes()) + 1) * tariff.getPricePerMinute()) + tariff.getPrepaymentAmount();
        }
        return 0;
    }