import java.util.*;

abstract class Student {
    protected int[] scores;

    public Student(int[] scores) {
        this.scores = scores;
    }

    abstract String calculateGrade();
}

class Undergraduate extends Student {
    public Undergraduate(int[] scores) {
        super(scores);
    }

    @Override
    String calculateGrade() {
        int sum = 0;
        for (int score : scores) {
            sum += score;
        }
        double avg = (double) sum / scores.length;
        if (avg >= 90) return "A";
        else if (avg >= 80) return "B";
        else if (avg >= 70) return "C";
        else if (avg >= 60) return "D";
        else return "F";
    }
}

class Postgraduate extends Student {
    public Postgraduate(int[] scores) {
        super(scores);
    }

    @Override
    String calculateGrade() {
        int sum = 0;
        for (int score : scores) {
            sum += score;
        }
        double avg = (double) sum / scores.length;
        if (avg >= 85) return "A";
        else if (avg >= 75) return "B";
        else if (avg >= 65) return "C";
        else if (avg >= 55) return "D";
        else return "F";
    }
}

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        if (!sc.hasNext()) {
            System.out.print("Invalid input");
            return;
        }

        String type = sc.next();
        if (!type.equalsIgnoreCase("Undergraduate") && !type.equalsIgnoreCase("Postgraduate")) {
            System.out.print("Invalid input");
            return;
        }

        if (!sc.hasNextInt()) {
            System.out.print("Invalid input");
            return;
        }

        int n = sc.nextInt();
        if (n <= 0) {
            System.out.print("Invalid input");
            return;
        }

        int[] scores = new int[n];
        for (int i = 0; i < n; i++) {
            if (!sc.hasNextInt()) {
                System.out.print("Invalid input");
                return;
            }
            scores[i] = sc.nextInt();
        }

        Student student;
        if (type.equalsIgnoreCase("Undergraduate")) {
            student = new Undergraduate(scores);
        } else {
            student = new Postgraduate(scores);
        }

        System.out.print(student.calculateGrade());  // ⚠️ no println, no extra space
    }
}