import java.util.Scanner;

class Main {

	public static void main(String args[]){	
		Scanner scan = new Scanner(System.in);

		int m = 0, f = 0, r = 0;
		while(true){
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if(m == -1 && f == -1 && r == -1)break;
			
			if(m >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
		}
		
		
	}
}	 import java.util.Scanner; Scanner java.util class Main {

	public static void main(String args[]){	
		Scanner scan = new Scanner(System.in);

		int m = 0, f = 0, r = 0;
		while(true){
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if(m == -1 && f == -1 && r == -1)break;
			
			if(m >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
		}
		
		
	}
} Main public static void main(String args[]){	
		Scanner scan = new Scanner(System.in);

		int m = 0, f = 0, r = 0;
		while(true){
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if(m == -1 && f == -1 && r == -1)break;
			
			if(m >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
		}
		
		
	} main {	
		Scanner scan = new Scanner(System.in);

		int m = 0, f = 0, r = 0;
		while(true){
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if(m == -1 && f == -1 && r == -1)break;
			
			if(m >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
		}
		
		
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int m = 0 m 0 f = 0 f 0 r = 0; r 0 while(true){
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if(m == -1 && f == -1 && r == -1)break;
			
			if(m >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
		} true {
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if(m == -1 && f == -1 && r == -1)break;
			
			if(m >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
		} m = scan.nextInt() m scan.nextInt() scan.nextInt scan f = scan.nextInt() f scan.nextInt() scan.nextInt scan r = scan.nextInt() r scan.nextInt() scan.nextInt scan 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 >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			} m >= 0 && f >= 0 m >= 0 m 0 f >= 0 f 0 {
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			} if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					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("A") System.out.println System.out System System.out "A" if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				} m + f < 80 && m + f >= 65 m + f < 80 m + f m f 80 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 < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				} m + f < 65 && m + f >= 50 || r >= 50 m + f < 65 && m + f >= 50 m + f < 65 m + f m f 65 m + f >= 50 m + f m f 50 r >= 50 r 50 {
					System.out.println("C");
				} System.out.println("C") System.out.println System.out System System.out "C" if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				} m + f < 50 && m + f >= 30 m + f < 50 m + f m f 50 m + f >= 30 m + f m f 30 {
					System.out.println("D");
				} 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("F") System.out.println System.out System System.out "F" {
				System.out.println("F");
			} System.out.println("F") System.out.println System.out System System.out "F" String args[] args
import java.util.Scanner;

class Main {

	public static void main(String args[]){	
		Scanner scan = new Scanner(System.in);

		int m = 0, f = 0, r = 0;
		while(true){
			m = scan.nextInt();
			f = scan.nextInt();
			r = scan.nextInt();
			
			if(m == -1 && f == -1 && r == -1)break;
			
			if(m >= 0 && f >= 0){
				if(m + f >= 80){
					System.out.println("A");
				}else if(m + f < 80 && m + f >= 65){
					System.out.println("B");					
				}else if(m + f < 65 && m + f >= 50 || r >= 50){
					System.out.println("C");
				}else if(m + f < 50 && m + f >= 30){
					System.out.println("D");
				}else if(m + f < 30){
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
		}
		
		
	}
}	