import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		}
		scanner.close();
	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		}
		scanner.close();
	}
} Main public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		}
		scanner.close();
	} main {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		}
		scanner.close();
	} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		} true {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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 = scanner.nextInt(); m scanner.nextInt() scanner.nextInt scanner int f = scanner.nextInt(); f scanner.nextInt() scanner.nextInt scanner int 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;
			} break; if(m==-1||f==-1) {
				System.out.println("F");continue;
			} m==-1||f==-1 m==-1 m -1 1 f==-1 f -1 1 {
				System.out.println("F");continue;
			} 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("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("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("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("C") System.out.println System.out System System.out "C" {
					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" scanner.close() scanner.close scanner String[] args args public class Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		}
		scanner.close();
	}
} public class Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		}
		scanner.close();
	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				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");
			}
		}
		scanner.close();
	}
}
