import java.util.*;
class Main{
	static Scanner s = new Scanner(System.in);
	public static void main(String[] args){
		int m,f,r;
		while (s.hasNext()) {
			m = Integer.parseInt(s.next());
			f = Integer.parseInt(s.next());
			r = Integer.parseInt(s.next());

			if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			}

			if(m+f>=80)
				System.out.println("A");
			else if(m+f>=65)
				System.out.println("B");
			else if(m+f>=50)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F");
		}
	}
} import java.util.*; util.* java class Main{
	static Scanner s = new Scanner(System.in);
	public static void main(String[] args){
		int m,f,r;
		while (s.hasNext()) {
			m = Integer.parseInt(s.next());
			f = Integer.parseInt(s.next());
			r = Integer.parseInt(s.next());

			if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			}

			if(m+f>=80)
				System.out.println("A");
			else if(m+f>=65)
				System.out.println("B");
			else if(m+f>=50)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F");
		}
	}
} Main static Scanner s = new Scanner(System.in); s new Scanner(System.in) public static void main(String[] args){
		int m,f,r;
		while (s.hasNext()) {
			m = Integer.parseInt(s.next());
			f = Integer.parseInt(s.next());
			r = Integer.parseInt(s.next());

			if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			}

			if(m+f>=80)
				System.out.println("A");
			else if(m+f>=65)
				System.out.println("B");
			else if(m+f>=50)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F");
		}
	} main {
		int m,f,r;
		while (s.hasNext()) {
			m = Integer.parseInt(s.next());
			f = Integer.parseInt(s.next());
			r = Integer.parseInt(s.next());

			if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			}

			if(m+f>=80)
				System.out.println("A");
			else if(m+f>=65)
				System.out.println("B");
			else if(m+f>=50)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F");
		}
	} int m m f f r; r while (s.hasNext()) {
			m = Integer.parseInt(s.next());
			f = Integer.parseInt(s.next());
			r = Integer.parseInt(s.next());

			if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			}

			if(m+f>=80)
				System.out.println("A");
			else if(m+f>=65)
				System.out.println("B");
			else if(m+f>=50)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F");
		} s.hasNext() s.hasNext s {
			m = Integer.parseInt(s.next());
			f = Integer.parseInt(s.next());
			r = Integer.parseInt(s.next());

			if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			}

			if(m+f>=80)
				System.out.println("A");
			else if(m+f>=65)
				System.out.println("B");
			else if(m+f>=50)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F");
		} m = Integer.parseInt(s.next()) m Integer.parseInt(s.next()) Integer.parseInt Integer s.next() s.next s f = Integer.parseInt(s.next()) f Integer.parseInt(s.next()) Integer.parseInt Integer s.next() s.next s r = Integer.parseInt(s.next()) r Integer.parseInt(s.next()) Integer.parseInt Integer s.next() s.next s if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			} m==-1||f==-1 m==-1 m -1 1 f==-1 f -1 1 {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			} if(m==f&&f==r)
					return;
				else
					System.out.println("F"); m==f&&f==r m==f m f f==r f r return; System.out.println("F") System.out.println System.out System System.out "F" continue; if(m+f>=80)
				System.out.println("A");
			else if(m+f>=65)
				System.out.println("B");
			else if(m+f>=50)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F"); 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)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F"); 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)
				System.out.println("C");
			else if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F"); m+f>=50 m+f m f 50 System.out.println("C") System.out.println System.out System System.out "C" if(m+f>=30) {
				if(r>=50)
					System.out.println("C");
				else
					System.out.println("D");
			}else
				System.out.println("F"); m+f>=30 m+f m f 30 {
				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" System.out.println("F") System.out.println System.out System System.out "F" String[] args args
import java.util.*;
class Main{
	static Scanner s = new Scanner(System.in);
	public static void main(String[] args){
		int m,f,r;
		while (s.hasNext()) {
			m = Integer.parseInt(s.next());
			f = Integer.parseInt(s.next());
			r = Integer.parseInt(s.next());

			if(m==-1||f==-1) {
				if(m==f&&f==r)
					return;
				else
					System.out.println("F");
				continue;
			}

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