import java.util.Scanner; 
public class grade{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int score = sc.nextInt();
        if(scpre<0 || score>100){
            System.out.print("Invalid  input");
        }
        if(score>89 && score<101){
            System.out .println("A");
        }
        else if(score>79 && score<90){
            System.out .println("B");
        }
        else if(score>69 && score<80){
            System.out .println("C");
        }
        else if(score>59 && score<70){
            System.out .println("D");
        }
        else{
            System.out .println("F");
        }
    }
}