import java.util.Scanner;

public class LetterGrade {

    public static void main(String args[]) {

        Scanner in = new Scanner(System.in);
        System.out.print("Please enter a score (0-100): ");
        int score = in.nextInt();

        if (score >= 90)
            System.out.println("A");
        else if (score >= 80)
            System.out.println("B");
        else if (score >= 70)
            System.out.println("C");
        else if (score >= 60)
            System.out.println("D");
        else
            System.out.println("F");

        in.close();
    }
}