using System;

public class Program

{

    public static void Main()

    {

        int x;

     x =int.Parse(Console.Readline());

        if (90 <x <100)

        {Console.WriteLine("A");}

        if else (80<x<89)

        {Console.WriteLine("B");}

        if else(70<x<79)

        {Console.WriteLine("C");}

        if else(60<x<69)

        {Console.WriteLine("D");}

        else

        {Console.WriteLine("F");}

    static void

    }

}