import java.util.Scanner;

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

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