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.print("A");
}
else if (score<90 && score>=80)
{
System.out.print("B");
}
else if (score<80 && score>=70)
{
System.out.print("C");
}
else if (score<70 && score>=60)
{
System.out.print("D");
}
else
System.out.print("F");
}
}