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");
}
}
}
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