import java.util.Scanner;
public class P2
{
    public static void main(String [] args)
    {
        int score = 0;
        Scanner in = new Scanner(System.in);
        System.out.print("Enter your score: ");
        score = in.nextInt();
        if (score < 40)
        System.out.println("F");
        else if (score >= 40 || score < 50)
        System.out.println("D");
        else if (score >= 50 || score < 60)
        System.out.println("C");
        else if (score >= 60 || score < 70)
        System.out.println("B");
        else if (score >= 70 || score < 80)
        System.out.println("B+");
        else
        System.out.println("A");

    }
}