import java.util.Scanner;

public class Main{
	public static void main(String [] args){
		Scanner sc = new Scanner(System.in);
		int m,f,r;
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			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,f,r;
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
		}
	}
} Main public static void main(String [] args){
		Scanner sc = new Scanner(System.in);
		int m,f,r;
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
		}
	} main {
		Scanner sc = new Scanner(System.in);
		int m,f,r;
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int m m f f r; r 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 while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
		} !(m == -1 && f == -1 && r == -1) (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.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
		} if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			} m == -1 || f == -1 m == -1 m -1 1 f == -1 f -1 1 {
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			} System.out.println("F") System.out.println System.out System System.out "F" 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 continue; if(m == -1){
				m = 0;
			} m == -1 m -1 1 {
				m = 0;
			} m = 0 m 0 if(f == -1){
				f = 0;
			} f == -1 f -1 1 {
				f = 0;
			} f = 0 f 0 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{
				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 && 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{
				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("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{
				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("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{
				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("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" 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,f,r;
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			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,f,r;
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			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,f,r;
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(!(m == -1 && f == -1 && r == -1)){
			if(m == -1 || f == -1){
				System.out.println("F");
				m = sc.nextInt();
				f = sc.nextInt();
				r = sc.nextInt();
				continue;
			}
			if(m == -1){
				m = 0;
			}
			
			if(f == -1){
				f = 0;
			}
			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{
				System.out.println("F");
			}
			m = sc.nextInt();
			f = sc.nextInt();
			r = sc.nextInt();
		}
	}
}