import java.util.Scanner;

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

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