9498 : 시험 성적

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();

		if(A>=90) System.out.println("A");
		else if(A>=80) System.out.println("B");
		else if(A>=70)System.out.println("C");
		else if(A>=60)System.out.println("D");
		else System.out.println("F");

	}
}