import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner S = new Scanner(System.in);
		while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		}
	}		
} import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args){
		Scanner S = new Scanner(System.in);
		while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		}
	}		
} Main public static void main(String[] args){
		Scanner S = new Scanner(System.in);
		while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		}
	} main {
		Scanner S = new Scanner(System.in);
		while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		}
	} Scanner S = new Scanner(System.in); S new Scanner(System.in) while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		} true {
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		} int m = S.nextInt(); m S.nextInt() S.nextInt S int f = S.nextInt(); f S.nextInt() S.nextInt S int r = S.nextInt(); r S.nextInt() S.nextInt S if(m == -1 && f == -1 && r == -1)
				break; m == -1 && f == -1 && r == -1 m == -1 && f == -1 && r == -1 m == -1 m -1 1 f == -1 f -1 1 r == -1 r -1 1 break; if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D"); (m + f < 30) || (m == -1 || f == -1) (m + f < 30) m + f m f 30 (m == -1 || f == -1) m == -1 m -1 1 f == -1 f -1 1 System.out.println("F") System.out.println System.out System System.out "F" if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D"); m + f >= 80 m + f m f 80 System.out.println("A") System.out.println System.out System System.out "A" if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D"); m + f >= 65 m + f m f 65 System.out.println("B") System.out.println System.out System System.out "B" if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D"); m + f >= 50 || r >= 50 m + f >= 50 m + f m f 50 r >= 50 r 50 System.out.println("C") System.out.println System.out System System.out "C" if(m + f >= 30)
				System.out.println("D"); m + f >= 30 m + f m f 30 System.out.println("D") System.out.println System.out System System.out "D" String[] args args public class Main{
	public static void main(String[] args){
		Scanner S = new Scanner(System.in);
		while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		}
	}		
} public class Main{
	public static void main(String[] args){
		Scanner S = new Scanner(System.in);
		while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		}
	}		
} Main
import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner S = new Scanner(System.in);
		while(true){
			int m = S.nextInt();
			int f = S.nextInt();
			int r = S.nextInt();
			
			if(m == -1 && f == -1 && r == -1)
				break;
			

			if((m + f < 30) || (m == -1 || f == -1))
				System.out.println("F");	
			else if(m + f >= 80)
				System.out.println("A");
			else if(m + f >= 65)
				System.out.println("B");
			else if(m + f >= 50 || r >= 50)
				System.out.println("C");
			else if(m + f >= 30)
				System.out.println("D");
		}
	}		
}