// Page 069

import java.util.Scanner;

class Prog_5_7
{
	public static void main(String[] args)
	{

			Scanner input = new Scanner (System.in);
			int score;
			score = input.nextInt();;
			switch ( score )
			{	case 5: System.out.println("A"); break;
				case 4: System.out.println("B"); break;
				case 3: System.out.println("C"); break;
				case 2: System.out.println("D"); break;
				case 0: System.out.println("F");
				default:
								System.out.println("out of range");
			}
	}
}