백준 9498

import java.util.Scanner;


public class Main9498 {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int score = sc.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");
		}
	}

}