import java.util.Scanner;
public class Main {
    
    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        int score = sc.nextInt();
        if(score >= 90 && score <= 100) {
            System.out.println("A");
        } else if(score >= 80 && score <= 89) {
            System.out.println("B");
        }else if(score >= 70 && score <= 79) {
            System.out.println("C");
        }else if(score >= 60 && score <= 69) {
            System.out.println("D");
        } else if(score >= 0 && score <=59){
            System.out.println("E");
        } else {
            System.out.println("F");
        }
    }
}