import java.util.Scanner;

public class Main {

	private static Scanner sc;

	public static void main(String[] args){
		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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(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 public class Main {

	private static Scanner sc;

	public static void main(String[] args){
		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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else System.out.println("F");
		}
	}
} Main private static Scanner sc; sc public static void main(String[] args){
		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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else System.out.println("F");
		}
	} main {
		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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else System.out.println("F");
		}
	} 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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else 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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else 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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else System.out.println("F"); m!=-1&&f!=-1 m!=-1 m -1 1 f!=-1 f -1 1 {
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(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>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(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>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					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||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				} m+f>=50||r>=50 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>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				} 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 System.out System System.out "F" String[] args args public class Main {

	private static Scanner sc;

	public static void main(String[] args){
		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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else System.out.println("F");
		}
	}
} public class Main {

	private static Scanner sc;

	public static void main(String[] args){
		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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else System.out.println("F");
		}
	}
} Main
import java.util.Scanner;

public class Main {

	private static Scanner sc;

	public static void main(String[] args){
		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){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else System.out.println("F");
		}
	}
}