import java.util.Scanner; 
public class Main { 
	public static void main(String... args) { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 
	} 
}
 import java.util.Scanner; Scanner java.util public class Main { 
	public static void main(String... args) { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 
	} 
} Main public static void main(String... args) { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 
	} main { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} true { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 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 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; if(m + f >= 80) { 
				System.out.println('A'); 
				continue; 
			} m + f >= 80 m + f m f 80 { 
				System.out.println('A'); 
				continue; 
			} System.out.println('A') System.out.println System.out System System.out 'A' continue; if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} m + f >= 65 m + f m f 65 { 
				System.out.println('B'); 
				continue; 
			} System.out.println('B') System.out.println System.out System System.out 'B' continue; if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} m + f >= 50 m + f m f 50 { 
				System.out.println('C'); 
				continue; 
			} System.out.println('C') System.out.println System.out System System.out 'C' continue; if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			} m + f >= 30 m + f m f 30 { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			} 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' continue; System.out.println('F') System.out.println System.out System System.out 'F' String... args args public class Main { 
	public static void main(String... args) { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 
	} 
} public class Main { 
	public static void main(String... args) { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 
	} 
} Main
import java.util.Scanner; 
public class Main { 
	public static void main(String... args) { 
		Scanner sc = new Scanner(System.in); 
		while(true) { 
			
			int m = sc.nextInt(); 
			int f = sc.nextInt(); 
			int r = sc.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'); 
				continue; 
			} 
			if(m + f >= 65) { 
				System.out.println('B'); 
				continue; 
			} 
			if(m + f >= 50) { 
				System.out.println('C'); 
				continue; 
			} 
			if(m + f >= 30) { 
				if(r >= 50) System.out.println('C'); 
				else System.out.println('D'); 
				continue; 
			}
			
			System.out.println('F'); 
		} 
	} 
}

