import java.util.Scanner;

public class targil1 {

//    //for creating main use : psvm
//    public static void main(String[] args) {
//        Scanner  s = new Scanner(System.in);
//        int age;
//        int height;
//        int weught;
//
//        System.out.println("Please enter age : ");
//        age = s.nextInt();
//        System.out.println("Please enter heught : ");
//        height = s.nextInt();
//        System.out.println("Please enter weight : ");
//        weught = s.nextInt();
//        int cala = height/weught;
//
//        if (cala>=0.5&&cala<2)
//        System.out.println("A");
//        else if (age<20)
//            System.out.println("b");
//        else
//            System.out.println("c");
//

    //   for creating main use : psvm
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int grade;
        int avg;
        int numTest;
        System.out.println("Please enter grade");
        grade = s.nextInt();
        System.out.println("Please enter avarge");
        avg = s.nextInt();
        System.out.println("Please enter num of tests");
        numTest = s.nextInt();
        double total;
        switch (numTest) {
            case 5:
            case 6:
                if (grade > 55) {
                    total = grade * 0.8 + avg * 0.2;
                } else
                    total = grade;
                System.out.println(total);
                break;
            case 7:
            case 8:
                if (grade < 55 && avg >= 8) {
                    total = grade * 0.75 + avg * 0.25;
                } else if (grade >= 55) {
                    total = grade * 0.70 + avg * 0.30;
                } else
                    total = grade * 0.8 + avg * 0.2;
                System.out.println(total);
                break;
            default:
                System.out.println("failed");
                break;
        }
    }
}