public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int score = Integer.parseInt(in.next());
        if (score > 85) {
            System.out.println("A");
        } else if (score > 75) {
            System.out.println("B");
        } else if (score > 65) {
            System.out.println("C");
        } else if (score > 55) {
            System.out.println("D");
        } else {
            System.out.println("F");
        }
    }