import java.util.Scanner;

class Main {

	public static void main(String[] args) {

		int m, f, r;

		Scanner stdIn = new Scanner(System.in);

		boolean flag = true;
		while (flag) {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			}
		}
	}
} import java.util.Scanner; Scanner java.util class Main {

	public static void main(String[] args) {

		int m, f, r;

		Scanner stdIn = new Scanner(System.in);

		boolean flag = true;
		while (flag) {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			}
		}
	}
} Main public static void main(String[] args) {

		int m, f, r;

		Scanner stdIn = new Scanner(System.in);

		boolean flag = true;
		while (flag) {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			}
		}
	} main {

		int m, f, r;

		Scanner stdIn = new Scanner(System.in);

		boolean flag = true;
		while (flag) {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			}
		}
	} int m m f f r; r Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) boolean flag = true; flag true while (flag) {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			}
		} flag {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			}
		} m = stdIn.nextInt() m stdIn.nextInt() stdIn.nextInt stdIn f = stdIn.nextInt() f stdIn.nextInt() stdIn.nextInt stdIn r = stdIn.nextInt() r stdIn.nextInt() stdIn.nextInt stdIn if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			} 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 {
				flag = false;
				break;
			} flag = false flag false break; if (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");
			} else {
				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("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");
			} else {
				System.out.println("F");
			} m + f >= 80 m + f m f 80 {
				System.out.println("A");
			} 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");
			} else {
				System.out.println("F");
			} m + f >= 65 m + f m f 65 {
				System.out.println("B");
			} 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");
			} else {
				System.out.println("F");
			} m + f >= 50 || r >= 50 m + f >= 50 m + f m f 50 r >= 50 r 50 {
				System.out.println("C");
			} System.out.println("C") System.out.println System.out System System.out "C" if (m + f >= 30) {
				System.out.println("D");
			} else {
				System.out.println("F");
			} m + f >= 30 m + f m f 30 {
				System.out.println("D");
			} System.out.println("D") System.out.println System.out System System.out "D" {
				System.out.println("F");
			} System.out.println("F") System.out.println System.out System System.out "F" String[] args args
import java.util.Scanner;

class Main {

	public static void main(String[] args) {

		int m, f, r;

		Scanner stdIn = new Scanner(System.in);

		boolean flag = true;
		while (flag) {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else if (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");
			} else {
				System.out.println("F");
			}
		}
	}
}