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