import java.util.Scanner;

public class Main {
	void run(){
		Scanner s = new Scanner(System.in);
		while(true){
		int f = s.nextInt();
		int m = s.nextInt();
		int r = s.nextInt();
		if(m == -1 && f == -1 && r == -1) break;
		if(m == -1 || f == -1){
			System.out.println("F");
			continue;
		}
		int sum = m + f; 
		if(sum >= 80) System.out.println("A");
		else if(sum >= 65 && sum < 80) System.out.println("B");
		else if(sum >= 50 && sum < 65) System.out.println("C");
		else if(sum >= 30 && sum < 50) {
			if(r >= 50) System.out.println("C");
			else System.out.println("D");
			}
		else if(sum < 30) System.out.println("F");
		}
	}

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

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

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

	public static void main(String[] args) {
	
		new Main().run();
		
	}
} Main
import java.util.Scanner;

public class Main {
	void run(){
		Scanner s = new Scanner(System.in);
		while(true){
		int f = s.nextInt();
		int m = s.nextInt();
		int r = s.nextInt();
		if(m == -1 && f == -1 && r == -1) break;
		if(m == -1 || f == -1){
			System.out.println("F");
			continue;
		}
		int sum = m + f; 
		if(sum >= 80) System.out.println("A");
		else if(sum >= 65 && sum < 80) System.out.println("B");
		else if(sum >= 50 && sum < 65) System.out.println("C");
		else if(sum >= 30 && sum < 50) {
			if(r >= 50) System.out.println("C");
			else System.out.println("D");
			}
		else if(sum < 30) System.out.println("F");
		}
	}

	public static void main(String[] args) {
	
		new Main().run();
		
	}
}