using System;
public class Program
{
public static void Main()
int scr;
scr = Convert.ToInt32(Console.ReadLine());
if(scr>=80 && scr<=100)
Console.WriteLine("A");
else if(scr>=70 && scr<80)
Console.WriteLine("B");
else if(scr>=60 && scr<70)
Console.WriteLine("C");
else if(scr>=50 && scr<60)
Console.WriteLine("D");
else
Console.WriteLine("F");
}