// editor1
import java.util.*;
public class Main{
    public static void main(String [] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        if(n>=90 && n<=100){
            System.out.print("A");
        }
        else if(n>=80 && n<=89){
            System.out.print("B");
        }
        else if(n>=70 && n<=79){
            System.out.print("C");
        }
        else if(n>=60 && n<=69){
            System.out.print("D");
        }
        else if(n<60){
            System.out.print("F");
        }
        else{
            System.out.println("Invalid input");
            return 0;
        }
    }
}