import java.util.Scanner;

class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int m, f, r;
		boolean sharp = false;

		while (true) {
			m = scanner.nextInt();
			f = scanner.nextInt();
			r = scanner.nextInt();

			if (m == -1 && f == -1 && r == -1)
				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");
		}
	}
} import java.util.Scanner; Scanner java.util class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int m, f, r;
		boolean sharp = false;

		while (true) {
			m = scanner.nextInt();
			f = scanner.nextInt();
			r = scanner.nextInt();

			if (m == -1 && f == -1 && r == -1)
				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");
		}
	}
} Main public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int m, f, r;
		boolean sharp = false;

		while (true) {
			m = scanner.nextInt();
			f = scanner.nextInt();
			r = scanner.nextInt();

			if (m == -1 && f == -1 && r == -1)
				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");
		}
	} main {
		Scanner scanner = new Scanner(System.in);
		int m, f, r;
		boolean sharp = false;

		while (true) {
			m = scanner.nextInt();
			f = scanner.nextInt();
			r = scanner.nextInt();

			if (m == -1 && f == -1 && r == -1)
				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");
		}
	} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int m m f f r; r boolean sharp = false; sharp false while (true) {
			m = scanner.nextInt();
			f = scanner.nextInt();
			r = scanner.nextInt();

			if (m == -1 && f == -1 && r == -1)
				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");
		} true {
			m = scanner.nextInt();
			f = scanner.nextInt();
			r = scanner.nextInt();

			if (m == -1 && f == -1 && r == -1)
				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 = scanner.nextInt() m scanner.nextInt() scanner.nextInt scanner f = scanner.nextInt() f scanner.nextInt() scanner.nextInt scanner r = scanner.nextInt() r scanner.nextInt() scanner.nextInt scanner 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 == -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 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 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 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 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 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.Scanner;

class Main {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		int m, f, r;
		boolean sharp = false;

		while (true) {
			m = scanner.nextInt();
			f = scanner.nextInt();
			r = scanner.nextInt();

			if (m == -1 && f == -1 && r == -1)
				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");
		}
	}
}