public class MyClass {
    public static void main(String args[]) {
        java.util.Scanner scanner = new java.util.Scanner(System.in);
        double baggagePriceOver20kg = scanner.nextDouble();
        double baggageWeight = scanner.nextDouble();
        int days = scanner.nextInt();
        int baggageNbr = scanner.nextInt();

        double baggageTax = 0.0;

        if (baggageWeight <= 10) {
            baggageTax = 0.2 * baggagePriceOver20kg;
        } else if (baggageWeight > 10 && baggageWeight <= 20) {
            baggageTax = 0.5 * baggagePriceOver20kg;
        } else {
            baggageTax = baggagePriceOver20kg;
        }

        if (days > 30) {
            baggageTax = 1.1 * baggageTax;
        } else if (days > 7 && days <= 30) {
            baggageTax = 1.15 * baggageTax;
        } else {
            baggageTax = 1.4 * baggageTax;
        }

        System.out.printf("The total price of bags is: %.2f lv.", baggageNbr * baggageTax);
    }
}